The NHS will treat anyone taken ill on UK shores. It did happen to me including minor surgery and no charges. I still had a UK address that at the time I was associated with. Is your employer providing healthcare? Don't be worried when in Qatar, the HMC (like NHS) is excellent and has clinics and hospitals all over the country. My wife had emergency surgery and it was exceptional, great care and service provided.
For any insurances - Life, Medical etc, that you take out in the UK has to be from a broker and underwriting that specializes in expat policies, one I know of is William Brown.

PM me if you have any other concerns or questions