UK woodland site limit for guns is 350fps, CQB limit is usually 328fps.
There's a few choices for an AK which are within your budget:
G&G RK-47 (http://www.zerooneairsoft.com/product_info.php?products_id=8790) probably the best of the bunch. Fairly good internals and a strong plastic body, with a couple of metal parts. You can get models without blowback for less, but the blowback is a nice touch. If you don't like it you can always get it disabled by a tech.
Any CYMA AK47 with a model number of CM.048 or above. These are full steel VFC (?) clones. CYMA internals aren't the best out of the box, they'll need a downgrade as out of the box they clock in at 380fps or thereabouts. They're really noisy OOB as they usually need the gears' angle of engagement corrected and the gearboxes are caked with machine grease inside, so they may need disassembling and cleaning. You could get any tech to do that at most sites if you aren't comfortable.
SRC AKs- don't bother. Piles of crap.
ICS AKs- if you can stretch your budget a little, they're worth it.
But before purchasing a RIF, you need to get yourself registered on the UKARA database. You can do this by attending 3 games in no less than 2 months, getting a marshal to sign off the paperwork you can get from most site offices, and then once the form is filled you hand it in and they'll put you on the database for a £5 or less admin fee.
You also need to be over 18 to purchase a RIF. If you aren't you can ask a parent/older brother/over 18 friend to purchase an AEG for you. If they don't have a UKARA defence or similar, 51% of the gun would have to be painted a bright colour before a retailer will ship it. Most online retailers will do a 2toning service for under £20. Zero One do it for free on selected guns though.