Buying a new mattress, advice appreciated please

Hi, I'm due to move back into my flat in a couple of weeks time and I need a new double mattress (although I would rather wait for the best mattress possible rather than impulse-buy one). I would like the best I can possibly get as I have a slipped disc (lower back) and trapped nerves in both my lower and upper back which causes pain in my knees, ankles, neck and shoulders. Unfortunately, I'm restricted to a budget of around 700GBP. I've previously looked through your website and checked reviews which not only highly rate the service you provide but also the mattresses available. I'm 5ft 5in tall and weigh 10 and a half stone. I have a bedstead with slats and on the whole prefer a firmer bed as I have a thinner/firmer pillow. I would prefer natural fibres. If you could provide me with any advice or suggestions on a mattress. Ultimately I love your Artisan 1500 or Tailored Pocket but I imagine this would only be possible through your outlet source.
I look forward to hearing from you.