I've encountered an odd pronunciation inconsistency with Samantha myself. There is a road on my way to work - Koehler Rd. Rhymes with "roller" as far as I know although I guess it could equally as well be pronounced more like "mailer".
In my travels, Samantha pronounces it like "keller". So I noticed in MapShare I can note an incorrect pronunciation. When I go to do that, it pronounces it for me first... and it says it correctly!
Don't know why it won't say it correctly while driving
My desire for a British accent is just a silly preference. I've read that people, even (or mostly) in the US, feel better when machines speak to them with British accents than American ones. Perhaps that's because we don't notice the little inconsistencies of machine pronunciation when the accent is a little foreign anyway. My wife's favorite ATM used to have a British voice. We were both disappointed when they changed it to American.
By the way I heard back from TomTom support that indeed, there is no British computer voice available because the 130S is not available in the UK.