Sorry, I know that you are in the UK (and I am in the US), but I would only buy from reputable shops like Nokia, Amazon, Dell, etc. Among them, who has the best price and shipping time? Return policy?

I would not go really cheap (ebay, some Chinese website, etc), as there is way too much risk involved. Risk to get a KIRF, damaged device, used not new, etc. etc.

UK: Did you look if a subsidized phone bought via a provider is more beneficial bottom line?