Which bed?

I'm female, 13st 9lbs, sleep on my side mainly and like to feel not much pressure on my hips and shoulders. Sometimes I lie on my back and like to feel flat (no hammocking!) I find that medium beds feel too soft when I lie on my back, but firm beds give me dead arms and pins and needles. I have a sprung slatted base on which to put the mattress. I bought a John Lewis Natural Collection Linen 4000 Firm mattress after trying different beds for about an hour in the store and, although it felt fine at the time, I can't sleep on it as it's too firm. JL won't exchange it which leaves me with a loss of about £800 so it's destined for eBay. The bedstead needs a mattress of about 25cm to look right, but that's a lesser consideration. Which of your beds might fit the bill please?