Main problem with going off grid in the UK is just the sheer cost of land... One isn't going to get change out of £100K for say 5 acres of woodland which is really the minimum you need for some basic privacy, and then you'd probably need a 4 wheel drive which are astronomically expensive! You'd want to spend £!5K on a second hand one minimum I think to make sure it'll last you a decade and isn't a total lemon.

Now I fully understand why Brits are coming to live in Spain and Portugal... I was checking prices in Portugal, they are ridiculously low even compared to Spain! And you got a piece of the forest, a small river crossing your property, and a house... Okay, the house is usually a ruin, but still... the prices that I saw were insanely low... Plus, I suppose that the climate is much better here than in the UK :)

Btw. was watching the UK show "Brits in the sun" and you can get some useful info about prices and different regions... :)