Here in East Anglia we're lucky enough to have a really well-respected specialist firm, Mattressman. In case you happen to be around here as well, their stores are located here:
http://www.mattressman.co.uk/stores/
Even if you're not, you can still buy online with confidence. Their reviews on TrustPilot are outstanding:
http://www.trustpilot.co.uk/review/www.mattressman.co.uk
Here's their range of double mattresses:
http://www.mattressman.co.uk/mattresses/double/
You'll see that all of the
really expensive ones use 'pocket spring' structure, so I suggest using the filters at the top of the page to select that structure type. That will produce 6 possibilities within your price range, with 3 of the higher-priced ones having a Comfort Rating of 7. (Alternatively, if you'll settle for 'reflex foam', rather than 'pocket spring', you can achieve the same Comfort Rating for around £100 less).
Note that, as long as you're on the UK mainland, delivery is FREE.
And no, I don't work for them ;-)
Chris