Ehlers danlos syndrome, and a very tiny budget

Hi,
I have ehlers danlos syndrome, where my joints slip in and out of socket, I bruise easily and have delicate skin.
My husband has a congenital spinal problem resulting in chronic back pain.
We have a budget of £100 ish right now, but are saving up as much as we can afford (only my husband is well enough to earn a wage)
Do you have any advice on how we can obtain a kingsize mattress that we can cope with? We urgently need to move to a bigger bed as our double is too cramped for us now.