I work at Halfords part time. I'd recommend a Garmin over the TomToms anyday. TomTom only really sell by name but we have a lot of problems - mainly silly ones and battery issues - with the TomTom units.
Garmin were selling sat nav equipment back in the 70s with satellite phones, etc. Their satnavs are more reliable, quicker and not quite as expensive.
Make sure you buy one with free lifetime maps! If not, it'll cost you approx £80 a year for map updates. You don't HAVE to update your satnav but it really helps...