Travel Insurance for Brazilian Partner to travel to UK

I am a UK citizen with a Brazilian partner planning to visit me in the UK for just under 90 days this summer.
Finding travel insurance for her has so far been perplexing, and the costs seem much greater than my UK travel insurance costs when visiting Brazil.
Can anyone advise what is the advisable ceiling cover for medical expenses  and whether they know of reliable travel insurance companies which do not charge over the top fees?
It seems that it is very unlikely that a traveller would need to resort to insurance for the worst of medical situations, but if the worst happened, such as in a road traffic accident, hospital fees could be potentially astronomic.
And suggestions greatly received.