Solar photovoltaic is never going to be a great idea in the UK. The physics (and they aren't going to change) don't add up.... the following is shameless ripped off from http://www.inference.phy.cam.ac.uk/withouthotair/c6/page_39.shtml... if you're thinking of Greenwashing I suggest you have a read)
The phyisical limit (i.e. it is impossible for it to be better - the Shockley–Queisser limit) for efficency of standard PV panals is 31%. In order to do better you need to split the light into multiple bandwidth widths - the best that anyone has done is 42% and the tripple junction limit (i.e. split into 3 bandwidths) is under 50%. The best you can buy are about 20% efficent at the moment.
Sunlight at midday provides 1kw/m2, as we're not at the equator we need to modify that to 600w/m2. The UK on average gets 34% of sunny daylight hours due to cloud cover. As a result you end up with 110w/m2 on average (see link aboves technical chapters for working)for a south facing roof.
So, lets say you installed 10m2 (which would be a lot) of the best panals you can buy, you're only going to get on average 2.6kWhs a day (110*0.2 = 22W/m2, 22x10x12hrs=2.6kW/day). The average consumption for a 3 bed house is about 10.6kWhrs a day...
Thermal pannals make a lot more sense.
