Looks like you'll have fun here in the UK.
I must confess, despite living here, i haven't been to half of the cities that your route plans out.
But i'll add to what dani had to say.
Bath is a nicer town than Bristol, though both are very pretty
Not been to southern parts of Wales really, but avoid Aberysthwyth - it's a real fucking dump :roll:
Isle of Man looks like it would be fun - i know when i get a decnt car, i'm going to take the ferry out there to lap the TT circuit :good:
Scotland is green - and fucking cold!
Newcastle should be good fun - i've been told that its a good night out there 8)
Leeds is a cool cosmopoliton city - lots of expensive shops. But don't miss out York - that too is a nice city.
Macnhester is the best

... and Liverpool is similar to both Manc/Leeds.
Avoid Stoke - its a bit of a dump too
Birmingham and Coventry - their accents will make you laugh - they still crack me up
Anywhere east of Cambridge is a dump - they have the highest suicide rates over in East Anglia (Norwich and such) - really

Not actually been to Cambridge myself, but if its anything like Oxford, then it will be a nice place - try to visit both if you can :good:
Have a great trip - i'd love to hear what you make of our sunny Isles
BTW - how are you planning on getting around