tomtom are a really good unit, get the 300 if you just want it for UK roads, although it doesnt have ireland on it. the 500 is slightly better as it has major roads of western europe, and you can link your mobile phone up to it via bluetooth and use it as a handsfree kit.
the 700 is top of the range, it is similar to the 500 but it is a hard drive unit not loaded on a memory card, it has full stret level mapping of europe.