I was looking at plain, non edge variety s7s' and came across a few deals which I thought were pretty decent..O2
Firstly on O2 you can get a 'Perfect' Like New S7 32GB for £360.00 and it comes with 12 month warranty. For this to work you need to take it on 'pay monthly', but you can cancel the airtime as soon as you receive your order. Check the tariffs, click 'show more' and then 'Pay for your device in full'. Select the £360 upfront and £14 pcm tariff. On arrival of your order, if you don't want to keep the airtime simply ring or livechat O2 and tell them why its not going to work, e.g. 'I can't get a signal in my house, but I want to keep the phone.' You shouldn't be charged a single day for the sim contract as long as you cancel the same day. I've done this before and it works :)AMAZON
Secondly, you can get it brand new from a third party amazon seller at just £419.99 which seems a very good deal. I can't vouch for seller but feedback seems good.
Also check the used the phones on Amazon warehouse
as I have just purchased a "Like New" in pristine packaging for £403 minus the automatic 20% discount applied at checkout = £322.40. Sorry, there was only one :p
These do change frequently so keep checking.