I bring Tylenol, DayQuil, Pepto Bismol- it' sold here but only behind the counter, I believe. I bring vitamins too, as I find they're cheaper in the US. Also Unisom.... I VERY rarely need it, but I'm grateful to have it around if I can't sleep/have too much on my mind.
Agree with the others that you can find a lot of substitutions or make your own. I love cinnamon sweets, like Big Red gum and cinnamon disks, so I bring those back. Twizzlers, seasonal/holiday candies that are hard to come by in UK (like the Reese's pumpkins in fall). Boxed mac and cheese. Can you see a pattern with the kind of food I bring back?

I so wish that I could bring back a big carton of International Delights iced coffee as well.
Bras. I'm a bit of a Bath and Body Works junkie too, so I usually end up bringing back some shower gel, cream, hand soap, maybe a 3 wick candle.