6s, if it's cheap and affordable now, sure. 6, no way - do they even sell iPhone 6 still? I thought the lineup is now SE, 6s, 7.
Question: Do you need to get it on contract with the phone, or are there prepaid, month-to-month or other BYO phone plans that are cheaper in the UK? I did the maths and it's not worth getting a phone on a plan in Australia unless you want to get a new phone every two years or you use a shit ton of calls and data. Yes, you get the phone subsidised but you're also end up paying for things you don't need.
I got off that wagon and just buy the phone outright, upgrade in three years instead of two, got a much cheaper plan that better matches what I use and I'm saving hundreds over the course of the three years.
The main issue is the amount of RAM. It's something that was already apparently after the first year, Safari tabs having to reload, opening the Camera app causing a lot of other apps to be unloaded, widgets being unloaded... While the CPU is noticeably slower than its successors, it's still works very well.
If it weren't for the RAM and me wanting to upgrade for a better camera, I'd probably consider holding on to it for four years before changing phones.