Stoop, This is available Nationally. In effect you are renting your roof to this company. They will cover the roof area with Photo Voltaic (PV) panels and then claim the government Feed In Tariff (FIT) of 43 pence per kilowatt hour produced by the panels (average around £1,500 per year) and they get that for the next 25 years ! The FIT is also index linked and will rise with inflation throughout that time.
You have to think about the "free electric" claims made by these companies, PV Panels only produce electric during daylight hours and obviously are most efficient when the sun shines, if you are at work most of the day, all of this power is exported back to the grid and you get nothing.
If you still fancy going ahead with this, consider investing in the installation yourself (£6.000 to £8,000 ) and you keep the FIT. Alternatively consider Solar Thermal where you will get hot water or a Micro Combined Heat and Power boiler (MCHP) which will produce electricity while the boiler runs and you will still get a FIT benefit, all be it at a lower level than PV.