If you compare a MC (monocrystalline) and PC (polycrystalline ) PV (photovoltaic) panel with the same full-sun efficiency in each case, there is very little difference in efficiency as light level falls, between the MC & PC panel.
For the same full-sun efficiency monocrystalline are slightly better in low light conditions but the difference is liable to be well under 10% of output in any given circumstances and other factors such as keeping the panels clean, will usually have a greater effect.
Monocrystalline PV cells are available in slightly higher efficiencies than are polycrystalline but the main effect that this has is that polycrystalline panels are slightly larger for the same power output. Higher efficiency cells usually command a price premium - which is only of benefit to the user if available space is limited and you want as much power as possible within a given area.
PC are up to about 15% efficient and
MC up to about 18%.
SunPower make "rear contact" MC cells that have the current collection wires on the rear surface and also have optimisations aimed at reducing reflections (which otherwise reduce available light) and dealing with off angle light slightly better than usual - all of which adds up to give about 23% efficiency in commercially available cells - but cost per Watt is higher and they will generally not be the best option in "normal" applications.
Modest gains can be achieved by edging your panels with low cost reflectors to achieve a very modest degree of concentration. 10% - 20% extra power is "not too hard [tm] to achieve this way. [3M make a reflective film specifically aimed at this application].
While it is very seldom mentioned or used, modest but useful gains in output can be achieved by cooling the panel somewhat. Whether this is worthwhile depends on available means of cooling. Water cooling works very well but is not usually practical, and air cooling is a possibly but liable to be pursued only by the "excessively enthused" :-). With the present low and falling cost of PV panels, just buying one more is usually more economical than trying to get more power out of a given number of panels.
It is important to note that available energy is related to light level and that cloudy or end of day outputs will be well down on peak outputs. While this sounds obvious, numerous solar product makers do not make this clear. THE most important requirement is a clear line of sight between the panel surface and the sun (or brightest spot in sky). On a full sun day, if direct sunlight does not fall on the panel the out[ut will be under 10% in most cases and perhaps as high as 20% if an exceptionally good reflective path is available.
A very bright cloudy day such that the sun location cannot be determined but the cloud is so bright as to hurt your eyes will give perhaps 20% - 30% of full output.
Sunlight through a window will give perhaps 80% of full output IF direct sunlight falls on the whole panel surface. Otherwise output will be poor, as above.
In low insolation areas (which would include the UK) it is vital that panels are mounted at the correct elevation angle, and changing this a few times per year will give you slightly better output. This is well covered on the internet. One excellent site is www.gaisma.com with London data and All UK locations as examples.
The diagram below from Gaisma's London page show the path of the sun with time during a day at midsummer (green), midwinter (blue) and now (early March, blue). Midday sun angle from horizontal is 60 degrees in midsummer, about 15 degrees !!! in midwinter and about 30 degrees at present. What angle you set your panels to depends on whether you can alter the angle occasionally throughout the year or intend to leave them at a fixed angle. Fixed optimum angle depends on whether you want maximum energy overall or maximum winter output or some other choice.