I completely understand your reasoning behind buying something cheap, a lot of people don't want to spend loads and / or want to see if they enjoy it (You should go to a skirmish a few times and hire, could get UKARA whilst you're at it?) It also gets pretty confusing with the choice of stuff and legalities (VCR Act, UKARA etc)
I realise it's only for target practice, but I don't think a spring rifle is worth spending £50 on. I doubt it will be sturdy and reliable either and it's definitely not a good starter gun.
It's probably worth mentioning we don't actually rate JustBBGuns very highly as you can see from thread in the 'Places to Buy' section
For that price I'd recommend taking a look at the DE tri-shot shotguns which are pretty decent and skirmishable as well.
In the UK the least you could spend on an electric airsoft gun would be £100+, something like a JG G36 as already mentioned, CYMA AK or G&G M4.