After speaking to a friend who works at PC World we are going to go for the TomTom One XL GB Traffic at £199.99 (OK the budget has gone up but my Mum and Uncle decided to pay a little more lol). We have found it at Amazon for £188 and the bloke (a manager) said he would match a cheaper internet price so we'll do that me thinks.
Thanks for everyone's advice (RE: which brand to go for) it's been very helpful