Actionhobbies have full metal KWA Glocks in two-tone for around £100. They'll need proper airsoft gas for the first use, due to coming "dry", but after that, a propane can adapter would suffice.
If you have to go two tone, then I'd recommend one of them. They're nice, they're heavy, the colour isn't *that* horrid and they kick nicely

I've done quite a bit of pistol shooting; both real and airsoft etc and I can honestly say that KWA are one of the nicest airsoft brands I've used
Alternatively, post an ad in the wanted section of the forum; people are always trying to shift two tone stuff, and a second hand KWA would probably fit into your price range a bit better.
I know you say it's just for fun, but the problem with cheap, is that they often aren't fun. You'll maybe use it once or twice, put it away for a month and come back to it to find it's eaten itself or something.