You cant beat TomTom, i used to sell them, and then went and worked for a Logistics Company where i have used pretty much every satnav under the sun...
TomTom is by far the best (excluding some very expensive truck or built in options) and you get brilliant value for money.
Used to be the case that TomTom had a price range that retailers could sell them for, so you didnt tend to find one place had it miles cheaper than the next. There was offers, and all that, but on the whole, prices are pretty similar.
Seems pointless buying one now, unless its a christmas gift, you'll find money off them in Jan Sales.
There are a wide range of tomtom devices, get the sales person to run through the differences and match what you need it to do, to what it does do, as usually people have features that they will never use.]
I purchased one from Holland recently, and was around £30 cheaper when p+p was taken into account...