mobilevaluer.com – the linked reviews will tell you which to avoid, typically the third or fourth “best price” seems to be the one to go for.
Yeah, I used O2 recycle a couple of times when I had a contract with O2.
Seems to be a third party “on behalf of” now. Someone posted a horror story on here a few weeks ago about them; the abridged version was that they were offered (figure), sent the phone in, were told it was ‘damaged’ and actually offered (figure about a tenth of the original offer), on the advice of the STW massive the poster responded saying “send it back, then” and they suddenly decided it was fine and paid out the originally agreed amount.
I’ve sent a few back to various places over the years and never had a problem. Couldn’t say who with though, I just used the MSE link in this post and looked at the last couple of pages of comments for each company till I found one that wasn’t chancing it.