Try Insure With if you have complicated pre-existing conditions, or Able2Travel, or STA. all are pretty good st offering cover and the increased premium might not be that high.
If you have complex medical issues, relying just on the EHIC is unwise, as you can find yourself severely out of pocket if you need to stay beyond your planned date of departure, need an attendant for the return journey, and have to pay what a national if that country pays (some have insurance-backed co-pay arrangements, and you may need to pay upfront, even if some/all is later refundable)