Mattress for chronic neck and trap issues

Hi, I have chronic neck and upper back pain. I am light at 11.5 stones male, but have broad shoulders and the firm mattress I have forces me to compress, leading to issues with tight scalines, traps and delt pain. As a side sleeper I read I should really have foam mattress, and medium would be best to allow me to sink into the mattress?
Could you recommend me a good no frills option? The top end of my budget would be £600 but i would prefer cheaper if something fits the bill.
Thanks