I used Staysure on several occasions
they are quite expensive, but I don't think you'll get much better or cheaper.
remember that they will not cover you while you have an undiagnosed condition, or are awaiting investigations
Also, if you are ill, have a change in medication, operation etc after you have taken out insurance, you must tell the insurer
if you make a claim, and they find you have not mentioned these changes, your insurance will be null and void.
a disaster if this happens on holiday in America, where you are advised to take out insurance for£10,000000 !