You can tint the back windows as much as you like. there are some restrictions on the fronts. however my previous car had every window (except the windscreen) tinted limousine black and had that for 2 years and never got stopped for it... and passed its MOT.

I paid £200 but that was 3 years ago and coming from London i cant recommend anyone for you.

cheers mate gives me a rough guide on how much to expect