I suggest that you question some of your assumptions.
Difficulties of off-grid.
Off-grid is HARD, and generally isn't worth it unless you are a long ways from the power lines. If you are looking at one acre, you probably aren't that far away.
Let's look at what goes into off grid:
Electricity
Grid connected solar means you size your PV to provide about as much power per year as you use. Off grid means you size it provide enough power to run essentials during the darkest, coldest time of the year. At my latitude and climate (Central Alberta -- about equivalent to Edinburgh) that worked out to be between 2 and 2.5 times the year round grid. For me, on a small farm I needed 20 KW installed capacity.
For off grid, you need to store this. Current thinking is you need to store 5 days power, but you really need to model this with your climate, and determine how many 'brown out' or 'fire up the generator' days per year you will tolerate. Battery tech is getting better and cheaper, but for me I figured I needed to store 80 kWh. PV arrays need to be kept clean. It's amazing how much power a big blob of seagull poop costs you.
Factor your batteries into it. Lead acid typically need to be replaced every 3-5 years, assuming an average of 30% discharge on a daily cycle. This can be increased with appropriate battery maintenance. Good chargers, inverters, and battery monitors. A controlled environment to keep them in. (Can't just be stuffed in a corner of the garage.)
You lose a fair bit of energy doing the DC->Battery Charger -> Inverter, so there is a win in using DC for your big power loads, and having smart enough controllers that you can tap power directly off the charger.
Grid connected solar,with surplus feeding the grid makes much more sense economically, and can be worth doing even if you only get credit on your bill for the surplus. (In some areas they have to buy the power back at the maximum price they are paying anyone.)
Water
Here, most 1 acre properties would be on city water. Going off grid for water either means being a fanatic about rain collection, or having a well or spring or creek, or trucking it in.
Heat
I figure you can get 1 cord of firewood per acre of woods sustainably. I've done it here for 20 years and the woods look pretty much like they always have.
Good house design will reduce your heating needs, and in much of Great Britain may eliminate space heating. Domestic hot water may still be an issue. The key for that is large storage tanks to match the time of hot water generation to times of use. Where a 30 gallon (120 liter) tank is suitable for most households that use electric or gas heaters, if you are heating hot water with the sun, I'd suggest putting in at least 200 gallons. If you are grid connected it may make more sense to have a smaller electric tank, and more PV cells.
You can truck in propane.
Sewage
There are various ways to dispose of waste, but here you can't use a septic field on properties under 10 acres. The closest you can come to off grid is to use composting toilets, reuse grey water in the garden, and try to reduce what goes into the pump out tank. I doubt you will be able to get permits to build without some provision for sewage disposal. Here, composting toilets aren't popular with building inspectors, and grey water usage is illegal.
Data
We're connected to the world net. Sure you can use your phone's data plan, but check how much data you use per month. For lots of us a fixed connection is worth it. We're connected via WiLAN to the world. Setting up that connection was pricey because we needed a 70 foot tower.
Building your own house.
Start with a trailer or RV (caravan, I think you call them) You can often get these dirt cheap used, and it gives you a place that is warm and dry while building.
Consider leichtlehm (light straw) Think of packing straw into forms that has been dampened with a clay/water slurry. It's finished with a coat of plaster.
Advantages: You start by building a pole frame or timber frame house. YOu get the roof on first. NOW you have a dry place to work -- at least when the wind isn't blowing. It goes up reasonably fast and the weights are fairly easy.
Note that leichtlehm is the basic technique used in Tudor times -- hence the dark beams with white fill, so if you get good at it, you may be able to work as a restorer. Flip that around: Work for a restorer, to learn how to do it right.
Straw bale construction is also viable. No frame needed for a single story building. Typical building envelope costs are on par to slightly higher than convention construction, but since more of it is doable yourself, you can pay with sweat instead of cash. Strawbale can also be done as infill on a pole barn house, which is worth doing if you have a rainy climate, or are skeptical about getting the roof on in a single dry season.
Cob is suitable for England's climate. A mix of mud and straw, it insulates better than stone, but not by a lot.
With any alternative building system, get the local building people on side first -- before you buy the land. Some are much more open to innovation than others. You may find that by being in another jurisdiction you have less hidebound building inspector.
1 acre isn't really enough for food independence. Raising enough food is difficult, and your neighbours may object to chickens and pigs. You may find a better deal 15 minutes further from civilization. The usual trade off is commute time versus land size. Here, (Alberta, Canada) 45 minutes is the magic number. We bought 80 acres and 2500 square feet for the half of what we'd have paid for a similar size house on a fractional acre lot in Edmonton.
Get your septic in early. You can run the caravan's waste to it.
Spend time designing your house to fit your climate.
Overbuild the air circulation system. A house that depends mostly on wood heater and/or solar gain for heating/ open windows for cooling is going to have heat or coolth in the wrong places a lot of the time. Having larger ducts, and bigger fans allows you to move warmth where it's needed, or bring a flood of cool air in during the night. Open concept living spaces can just use a ceiling fan -- about 12 watts on slow speed. Bedrooms can be considerably chillier than main living rooms. Monitor humidity though. High humidity can lead to mildew. Look at mildew resistant paint.
If you are going off grid or near off grid, there are other changes:
Plan for multiple electrical systems. Lots of caravan equipment is designed to work on 24v DC. Anything you can use straight off the battery avoids the inverter. In some cases you can run it directly from the PV cells, but this requires some smart circuitry.
You aren't going to get it right the first time, so make ALL your utilities accessible. Put wiring in raceways disguised as chair rails. Where surface mount wiring isn't workable, put in conduit so 6 years from now you can fish another wire pair to the living room. Wall plumbing has a closet on the other side with a removable panel. Drains are accessible their full length, and have lots of cleanout ports. Keep the grey water separate from the black water. You can use the greywater in the garden, or get two uses once as grey, once as black.
Keep the roof simple. Simple roofs (No valleys) are less likely to leak, and have simpler setups for collecting rain water.
A metal roof puts less junk than a asphalt shingle roof into your water.
Make the roof bigger than the house. Roof is fairly cheap, especially if it's simple. It keeps water off the walls, and provides storage for wood, tools, empty canning jars. It's also a place for the kids when it's raining. Lots of farm jobs are more pleasant if you can be outside, but dry.
Plan your house for using wood/peat for heat in colder months.
Do your homework. Become the local expert.
