The take away here, and this is really true of any country, not just the UK, is that it's a bad idea to suggest you're doing anything that could be construed to be paid work. I can virtually guarantee that if I turned up at the US border on a visa waiver and said I was a musician playing a few gigs they'd send me off for a secondary questioning at the very least. I wouldn't be surprised if I was refused entry.
The return ticket doesn't really factor into it: whether you're entering the UK, US, wherever - generally if you're going in on a tourist visa / waiver you can't work. And unless you have some documentation indicating your gigs are unpaid, you've only got your word...which isn't worth much.
The return ticket doesn't really factor into it: whether you're entering the UK, US, wherever - generally if you're going in on a tourist visa / waiver you can't work. And unless you have some documentation indicating your gigs are unpaid, you've only got your word...which isn't worth much.