As a point of interest, remember that sat nav is not 100% in true accuracy. In my location, we have witness recently the 7th juggernaut lorry rescue. The latest incident was a Polish trucker who had made is way from Poland safely, only to find that the end of his journey resulted in the lorry becoming jammed between two buildings. Luckily all the drivers have been very competent, as very little damage as been caused, to either building or vehicles. Still costs a small fortune and a few hours of heavy equipment rescue time though.
Tom Tom One is a good unit. See one on demo at your local Halford's store perhaps.