Went to ASDA in Great Bridge (West Midlands) yesterday, where they had a whole section of Pepperidge Farm including Milanos (regluar, mint and raspberry), Pirouettes, cookies (soft baked and chunk), Brussels and Goldfish (called Finz, although they were not with all the other items, but with the other cookies). If you can't find them in a local store, they're available online.
While at ASDA, I noticed lots of other new American items: RC Cola has been introduced for the first time. There was also 2 liter bottles of Cherry 7Up (only ever seen that in cans before). Starbucks bottle Frappucino. New double stuff Oreos (but also milk and white chocolate covered Oreos, which they've had for a really long time). Also, for people that know the Sams Club brand in the states, there was diet Dr Thunder, which I haven't seen before. There was a whole new refrigerated Polish section where there was kielbasa and pierogies. These are just the things that I noticed, but there might be lots of other new stuff that I didn't see.
I also spotted some Reese's pieces in Sainsburys last week.