Yes. Account for as many externalities as possible.
Environmental Impact Cost
Calculate the offset costs for both options, so that they would both be equally Carbon Neutral.
- The flight, as you've calculated is £100 + £10 = £110
- The train is about 40Kg CO2 so £170 + £1 = £171
Perhaps we can assume that CO2 footprint of getting to the airport is the same as the CO2 footprint of getting to the railway station so they cancel each other out.
Opportunity cost of travel time
The flight time is 01:45, the train duration is about 07:49. You have to put a price on your leisure time, personally my price is £15 per hour, its about the same as my labour rate. Other people may value their leisure time more or less.
So for the flight I'm using up £26.25 of my leisure time. For the train, it would be £117.25.
Adding these onto the CO2 and ticket costs
- Flight £136.25
- Train £288.25
Now both options are Carbon Neutral and we've accounted for the time taken. Now think about your job, where the money comes from, when you commute to work, you might generate CO2 if you travel by car, or by train, or electric car. Your job might generate CO2 just from running computers or heavy machinery.
Which ever travel option you chose for the trip to Toulouse has to be paid for by your labours, which generate some degree of CO2. Even if you offset the CO2 for your labour, it still means more labour to pay for it. To minimise the amount of labour you can chose the cheapest travel option, that is flying.
You would have to do twice as much work to pay for the train journey.
On the other hand. If you can somehow persuade your employer that you can do work on the train journey and get paid for it, then that throws the equations the other way.
Although the basic principle remains, to account for as many externalities as possible, convert them into some financial sum, and apply it.
Some people might argue that the CO2 cost (£20 per tonne) is a bit low. That's fine we can adjust our sums accordingly. The train is twice as expensive as the plane, so even if the Carbon Tax / Offset Cost of CO2 doubled, the plane might still be the better option. If the CO2 price increased sixfold (to £120 per tonne) then it might tip the balance, but society would be very different already in such a scenario.
Some people might argue that the plane pollutes the atmosphere and so has a greater cost. That's fine too, we can look up what that cost is and apply it. But also establish the precise CO2 footprint of the energy source for the train, is it diesel or electric, nuclear electricity or coal.
You can go on for ages, adding more and more marginal costs to either side, but it probably won't affect the outcome that the fast cheap flight is better than the slow expensive train, and the environmental impact is cheap to pay for.