I have a similar query.
My girlfriend and I work outside the UK, but our perminent residence is in the UK. We haven't been home in over a year due to work and want to take a 3 week vacation in the USA. Our work insurance will not cover us when on personal travel, and we are looking for insurance for the 3 weeks we will not be covered by our employer. Any pointers on who would provide such a cover as every option I have looked at requires that the journey originates in the UK.