Last week I bought, from a very reliable local repair man .a good condition iPhone 5s, 16 Gig, for my wife. I paid £150 plus £10 for a tough screen cover and case. It was, importantly, unlocked. Several of my family have switched to Giff Gaff as a SIM provider. They do a rolling monthly contract - a "goodybag". I have a £10 a month one with 500 minutes (I never use that amount), unlimited texts and 1Gig downloads - again as I mostly use that part on wi fi, I find that plenty. I could probably get away with a £7.50 a month package with 300 mins. Giff Gaff use the O2 system so one could get away with an iPhone locked to them, but I prefer an unlocked one. -Do ensure it is genuinely unlocked or Apple may block it at the next Ios update if it is "Jailbroken"
Game also offer such phones with a 1 year guarantee.