I don't know if this makes sense, but...
In America we say 'uh' and 'umm' and that is how we write it out. It seems like British writing uses 'er' and 'erm'
Is this because a Brit saying 'uh' sounds like 'er' and 'um' sounds like 'erm'? If you say 'uh' out loud (and your american) it sounds like a someone from the U.K. saying 'er'
I don't know if that makes sense to anyone but me, (or maybe it is something everyone already knows) but I pondered it today.
edit: I was reading Harry Potter