maybe worth hiring a gun on site rather than buying one if you've not played before. Epsom has hire guns and they will be far better than what you can get for your budget. They will also not be 2-tone which is a big plus, especially in the tunnels where you want to be as stealthy as possible (it is bloody dark in there). Go once and hire a gun, if you enjoy it then maybe think about buying one, like your first post states you don't want to waste money and hiring is far far cheaper than buying even a £80 gun.
None of us want to put you off, and we don't enjoy saying you need more money than you currently have, but it is the truth. If you hire for a few games over a couple of months and then save any allowance/wages/etc you could get hold of enough to buy one of the guns we all have suggested. Buying a cheap gun is a waste of money, when they break (not if they break) they are dead, a proper airsoft gun can be repaired and upgraded etc and the G&G Combat Machine would never embarrass you on range or accuracy even in woodland!
If you live near Epsom, then Dogtag Airsoft, Holmbush (near Gatwick/Crawley) is not far from there so in the future you may want to try that and any of the many other sites not too far from there in the future.