FWIW, I always use CeX (with the confusing website
uk.webuy.com) to both buy
and sell phones and laptops.
They appear to be an honest company, as far as I can tell their business plan
is based on offering a decent service to customers rather than on ripping them
off; they offer a 24 month warranty (which I've used several times and it
works); if you order from the website, they deliver often the next day, and
within 14 days you can return it for any reason - including to your nearest
shop - which is incredibly convenient.
My £0.02.
Ben