Laura, the GPS and software I use is for a PDA, so possibly different to what you are looking for. However, the program menu gives options of how far ahead of a turn you start getting warnings. Too far ahead is a nuisance in a built up area and not far enough isn't helpful at motorway speeds. I'm guessing that other GPS units would have the same facility.

From what I've seen British multi-lane roads are well signposted so there shouldn't be need for total reliance on GPS warnings. Also I've found British drivers more courteous than Australian and more likely to allow you to change lanes.

I'll be hiring a TomTom and driving round UK with it for the next month so will put these ideas to the test.