Last summer, when I left the school I was at in Shanghai, I had no problem changing the res permit to an L visa while I got my affairs in order and get set up with the new school that my employers are partnered with.
The only problem with doing this, is the L visa shows up as a 0 entry (extension) and this can make it difficult to get another L visa later in Hong Kong if you have too many of them.
Too many being an arbitrary number that the visa clerk seems to decide on a daily basis.
But if you are planning on heading back to the UK and only taking the occasional trip back this would not be a problem for you.
Cheers.