Hi Kitchu,
Welcome to your this forum.
I applied for a International license when I passed in HK. It's cost around HK$80.- quite cheap. I think you can apply one in UK, maybe you can go to British driving license website to check how to apply. I guess no need to take any driving test here as I know they'll recognise HK license.
Besides, as I know, it should be free on NHS.
Hope this helps!
Jellybeanyd