We had this challenge too when we got here because we liked to feed our kitties a high quality grain free food from the US (Taste of the Wild was one of their favorites). Your best bet is to go online - can recommend zooplus for: a) fast service and b) large selection and c) free shipping over x amount so your heavy food arrives at your door (its pretty low threshold too - like 50 UKP only). We have used them in both the UK and Sweden and its been great!
Don't know about dog food, but it is easy to get smaller packets to try food at least,, before you spring on a giant bag only to find your pets won't eat any of it. They do carry Taste of the Wild and Orijin, which you can also get in the US, if you recognize those brands. We tried our cats on grain-free, high value Purizon and that was Midge's new favorite food after eating primarily TotW for months! That cat seriously cannot get enough of that chow. Probably her favorite meal after the fresh north sea herring
farmor (grandma) buys for them once a month.
For wet they have been ok with Gourmet Perle, and Miramor Ragout Royale has been a firm favorite too. Ours like only chunks in super gravy, so you may have better luck if yours like pate - or at least a wider variety. Cosma wet has been good (either Thai or Normal) but their freeze dried snackies in the tube were a mega hit with our boy Martin. Midge can't be bothered as much, but she likes the hearties treats from Smilia (but not the dry food, frustratingly enough). I have heard good things about Lilys Kitchen, but since its pate we haven't tried it.
Not sure how into the whole grain vs grain free or wet vs dry food debate you are into, but I did ask this similar question on a pets forum, and got a lot of replies. Have a look:
http://www.petforums.co.uk/cat-health-and-nutrition/348340-just-moved-overseas-no-clue-about-brands.html
