Don't buy any of the shit from "BB gun" websites.
You will need a UKARA defence to buy a realistic looking gun. As the others have said, G&G Combat Machine is the best to go for. We're always recommending em, pretty much anything cheaper isn't worth it but G&Gs are really good, especially for the price compared to more expensive ones.
Pro Airsoft Supplies and Colchester Airsoft did sell a transparent (clear receiver, stock, grip and handguard IIRC) for £100 and a starter pack including good battery and charger, BBs, shitty mask for £120. Well worth it but sold out now I believe.
Check Airsoft World / Airsoft Zone / Zero One for a better selection G&G guns. They're all pretty well priced and a thousand times better than the £70 gun which will be fully plastic inside and out. G&Gs are fibre reinforced polymer, Taiwan made, they have great hop rubbers and so on.
Prices start at £118 for a G&G CM16 (same one as the transparent one I mentioned) up to about £200 in the CM range.
They'll all perform practically the same, but the externals are very different so choose which suits your budget and the look you like.
There's also models with a pneumatic blowback (moves a little plate in the chamber, where the ejection port is, sort of improves the sound a bit) metal versions etc.
Oh and from what I've heard Zero One's two-tone service isn't very good though. Apparently it's poor quality paint and takes a long time. Explains why it's free I guess. On the other hand you do get a free skirmish voucher though which is good.
I suppose you could look at it that worse quality paint will wear off easier and by that time you might have a defence? Not sure what the finish is like though