Go to the actual UK Apple webstore where I have just recently ordered my iPod from. Free P+P, you can add a 2 lined message on the back of the iPod for free (well I know mine you could), and the U2 iPod is there.
If you do want it slightly cheaper (like £5 cheaper) then search for U2 iPod on Kelkoo.co.uk. It searches loads of internet stores to find you the cheapest one. However. Some sites say that have stock, but you click on the actual website and it says they don't...BE CAREFUL!
This is why I ordered directly from Apple as at least its a reputable website!