Your mattresses compared to the John Lewis Natural Cotton 4000

Hi,
My wife and I have been looking at buying a new mattress and so far we have both liked the John Lewis Natural Cotton 4000. We tried most of the John Lewis Natural range and felt this one was the most comfortable in terms of level of firmness (I think we prefer medium a bit on the softer side), I have been told that they use different spring tension through the various mattresses within the range hence the firmness can vary.
For context I weigh just below 11 stones and my wife weighs 8 stones.
I wanted to ask you, firstly what level of firmness we should be looking for in a mattress (we both tend to sleep on our side or front). Secondly we have a budget of around £700, can you recommend any mattresses that may be a good match or is the John Lewis cotton mattress likely to be the best within this budget.
Thank you
Kiz