Are you specifically looking for 'unlocked' phones or do you want ones that are locked to a particular network. (Remember that you've got to pay for the use of the phones, as well as the original cost!). 'Locked' phones are usually far cheaper.
I've been toying with the idea of getting a new phone and I've been looking at this:
http://www.phoneshopbysainsburys.co.uk/pay-as-you-go-phones/samsung-galaxy-j1.html
(It's got very good reviews for phones in that price range).
If I get it, I'll probably go for the £12 per month rolling bundle offered by Sainsbury's. (800 mins, 5000 texts, 2Gb data):
http://www.phoneshopbysainsburys.co.uk/bundle
However, as that's a little beyond your stated price range, this might be of interest to you:
http://www.phoneshopbysainsburys.co.uk/pay-as-you-go-phones/refurbished-phones/samsung-young-2.html
or possibly this:
http://www.phoneshopbysainsburys.co.uk/pay-as-you-go-phones/samsung-galaxy-ace-black.html