Hey there, welcome to the forum!

You'll find quite a lot of discussion at this thread. The summary is basically:

It's not clear. Nokia have definitely stated it's coming to Maemo, but there are contradictory remarks about the N900 -- one person said definitely yes, and another that it wasn't on the roadmap, but might well at some point down the line.

So far Nokia's sat nav release has been global, so there is every reason to expect that if or when the N900 gets it the UK will be covered.

My personal take is that Ovi Maps on the N900 is clearly still a work in progress, and Nokia are certainly improving it, so fingers crossed.