Whenever I go to the UK, the first and last place I stop to shop is Boots, the Chemist. There's no real equivalent in the US that I know about - none of the major drugstore chains like Rite Aid or even Duane Reed in NYC come close. I just read this morning that Boots products are coming to all Target stores and 450 CVS and 350 Shoppers Drug Stores. I am sure that they won't have the full range of Boots' products but if they even have a small percentage of their makeup, I will be one happy gal! 
Boots' No 7 line of beauty products is made by the same company that makes Chanel products. Their colour range is great and I love the way the make up goes on. Their Protect and Perfect serum got a huge boost after Professor Lesley Regan proved on BBC2's Horizon last month that it actually did what it claimed and it is now sold out in the UK. You might be able to find it on eBay for about three times the original cost in GBP, which means about six times what it would cost in the US.
Another great thing about Boots, and most other UK stores, is their loyalty program, which puts any in the US to shame. I sometimes got a point per £ spent and then could sometimes redeem this for a point per pound! My other favourite item at Boots is the "Joy RIde" anti-motion sickness medicine. Anything that's got a name like that has to be great!
Boots also carries premium labels including Chanel, Dior, Lancome and Clinique. They also carry a limited range of home accessories, which were always very stylish. You can buy anything from Boots, from car insurance to candy... I can't wait to see what Target has.
P.S. That wacky woman you saw doing the happy dance in the aisles of the Target this morning was me! Thrilled that I can get all of the things I love on this side of the pond!