My girlfriend stayed in a UK hospital for a week last Christmas with a kidney infection. The bill came to thousands but luckily she took out insurance with a Thai company before she came and they were very efficient and covered everything.
I found the main thing the UK government are looking for is proof that your girlfriend has something to go back to Thailand for ,such as Job, property, land,Family commitments etc and that you are able to support her while here in the UK.
I found the process relatively easy to be fair and not as hard as many make out