Who offers travel insurance that covers a holiday in your country of citizenship?

I had similar insurance issues but within different countries (I work in one country and live in another) This seems to freak out insurers. I found a solution in American Express. In my case it was sufficient to take the Amex Gold Flying Blue credit card to get insured. The annual fee is quite high, but if you consider the additional "free" travel insurance, it is okay.

I explicitly asked the salesman if my status of a border work would make my insurance invalid, which is the case with many other competitors. His response was that American Express is an international brand, so they have experience with this.

You might also want to check other credit card companies, to see if they might have that addition in your country of residence.

The fact that you say "the trip must start and end in the UK" makes me think you are looking at British insurers. Instead you should look at insurers in your country of residence. I'm a British Citizen living in Canada and I have had no trouble finding insurance to go to Britain. Most insurers don't care about your citizenship, just your residence (unless your citizenship gets you coverage of course, which it doesn't in this case).

If you are living in the EEA then you may be covered, or you may be able to get it cheap. Anywhere else they will happily sell you insurance just like a local citizen.