When I visited NY I took back cupcakes for friends from Buttercup and had no problem getting them through customs - and when I told the cabin staff they were cupcakes they put them away safely for me also (Maybe I was lucky!)
We dont have things like black and white cookies over here so maybe those as I keep reading NY is famous for them!
Have a great time in London - wrap up warm!
I bought (online) a wonderful Stollen last year as a food gift, a fruit cake either filled with a cranberry mix or with a raisin mix. It made a great gift as it tasted wonderful and it looked great. It may have been the highlight at the table. :-) It is also very sturdy so it should survive a cross-Atlantic flight.
The Stollen is made by Sterntaler Bakery (www.sterntalerbakery.com). I got the recommendation from somebody at Chowhound.
Have fun in London!!