I have to use Cetaphil too, and it is fairly hard to find here. In practice I've found very few Boots that actually stock it, although your local one may order it for you if you ask. (If they do have it, it won't be on the regular shelves, you have to ask the pharmacist for it.) You may be stuck ordering it online.
I tried using the Boots Simple cleanser and it is really not the same, my skin definitely suffered.
I don't know where you live, but in London, the only two places I've found that reliably stock it are Garden pharmacy in Covent Garden, and John Bell & Croydon behind Selfridges near Oxford St.
If you can bring it from the States, do so! It's about 6-7 pounds a bottle here.
(If you've never used cetaphil you may be wondering what the big deal is... but for many of us, it really is the only thing we can put on our face! It's a godsend.)