If you type 'health insurance' in the search box top right of your screen, it will bring up many older threads on this subject. They are useful as they show clearly all the different things that can cause the insurers to increase their premium.
It depends so much on your current meds, where you are going, and when you were last in hospital - that's a big issue - I couldn't get insured for copd at all within 6 months of a hospital stay (I went without insurance, but it's not to be recommended!)
The BLF have lists of Insurance Companies who are prepared to quote for medical conditions. It's a pain, but I found it better to phone companies and speak to someone as ticking boxes on the internet never came up with a sensible quote.
I found the British Insurance Brokers Association to be very helpful. Someone recommended them to me and they got me insurance with Worldwide Travel at a much better price than anyone else came up with.
With regard to oxygen abroad, I can't help as like Jimmy, I've got my own concentrator, but the BLF also have fact sheets with advice on holidays abroad and how to organise your oxygen supplies.
Best of luck with this, Jan
British Insurance Brokers Association - biba.org.uk