You are correct, that you can pre-order the PhotoPass CD for $99.95. For more info, click
HERE. If you purchase the PhotoPass CD after your vacation (or "holiday", as you say across the pond ;-) ), the price is $124.95. These prices are as of Oct 2008, and are subject to change. So, pre-paying saves you 20%.
It may be helpful to know that it does have a moneyback guarantee, if for any reason you decide you don't want to purchase the PhotoPass CD after you get home (you have two weeks from when you return from "holiday").
Will it work in the UK? Yes, since the PhotoCD is simply a computer disk with image files (.jpg), you should be able to use it on any computer in the UK. It is not a video/movie DVD.
Hope that helps. ºoº