Thanks again everyone for all the advice and recommendations, it's genuinely appreciated.
Of course, if I could grab a GBBP, gas and possibly a spare mag for £60 (£70 at a hard push), that’d be awesome. I had a look on 18Airsoft and I do see some Double Dell pistols around £40, plus £10 - £15 for a spare mag, plus £10 for 300g gas, without shipping that's within budget but I've heard DD aren't great.
I get that AEPs aren’t the top performers compared to gas pistols, but given where I’m at financially, I’m just after a cheap secondary that’s sturdier than a £25 spring pistol and doesn’t have ongoing costs (like gas). I’ve got a bit of birthday money (£60) left over, and I’m not in the most stable financial position right now, so the cheap AEP as a one-off cost seems like the best way to feed the airsoft addiction and still feed myself.
A 300g green gas bottle costs around a tenner, which is half the cost of a day's airsofting. Granted, I’m not sure how many mag fills I’d get from a 300g bottle (guesstimate of 30, found here, though the experiment doesn't seem to have been fully completed), but for now, I'd rather spend my cash on actual gameplay rather than consumables. (I already have a metric assload of BBs, so that’s not a recurring cost currently.)
Once I’m more financially stable, I do plan on getting a gas pistol, so having an AEP as a secondary would also come in handy for lending to friends if they don’t have gear or a secondary.