Just ordered a Lumia 640 from O2 for £70 on PAYG. Should take my Giffgaff SIM card OK (shared network) and I've got 14 days to return it if it turns out to be a lousy buy.
In the end it came down to a mix of low cost & size that made me go this way - £70 is a relatively small gamble for something I'm not sure about (Windows on a phone). The other options were a Moto G4 plus - great screen, nice camera, OK battery life, huge piece of plastic, Moto E3 - smaller & neater but laggy when actually *doing* stuff & not just flicking across desktops, iPhone 6 - much smaller and neater (IOS10 is enormously better than IOS6, which completely turned me off) but also relatively expensive and with poor battery life.
I'll report back whether the Lumia experience is good or bad later.