The best way is to go to a bedding shop and try them out. Assuming you do not need anything special (e.g. orthopaedic etc) and you can afford it - try pocket sprung circa 1200 springs.
However, a good bedding company will usually recommend that you don't simply put a new mattress on an existing base. You might wish to consider a complete replacement. You should be able to get your old one taken away but there may be a charge for this.
trt, here are the Which Best Buys:
Silentnight Classic 1200 Pocket Deluxe circa £400 score 72%
Ikea Hasseng circa £500 score 72%
Ikea Hovag circa £180 score 70%