First make sure you are looking at the right county, i.e. where it is NOW, not where it was when your ancestor was buried there. This is particularly the case on the outskirts of what is now Greater London, formerly Kent, Surrey, ... When you add a birth or death location (maybe also when creating a cemetery), there is an option to do a search on the town name; try that.
In my, albeit limited, experience, UK towns have been pretty comprehensively added to the database, but not always where you first look.