You can do whatever you want on the rear windows, limo tint is legal (no different from driving a van I guess).
The law now states that you can't tint the front windows however although off the record most police aren't bothered about light front tints (I've never once been stopped in over 3 years)
If you haven't got £200 to spend on tints then I'd recommend saving up until you have. It's a nightmare of a job to do and best left to the professionals IMO. I tried to do my T-bar glass a while back and made a complete mess of it, waste of £20 worth of tinting film.