I am American and live in Texas and in my neighborhood, we have a large diverse population (there's a British/Indian shop outside my estate) so it's possible to get most UK food stuffs.
If someone were to bring me a gift, I would really love a Christmas ornament that's got a sort of UK theme to it. My mother-in-law buys me a keepsake like a robin or a postbox and one year she sent a hand-painted ornament of a specific English Heritage home that she enjoys. I especially enjoy fashion, food, interiors magazines and my husband always requests a photography one or history and cars. Weirdly, I love receiving a tea towel with a scene because I do use them and I always think about the giver when I do.
If any of your friends are drinkers, they may appreciate a specific bottle that can only be found in the UK or at duty-free. When I lived in England, my American girlfriends would ask for specific perfume found in UK shops so it may be worth asking your friends.
Recently, a UK friend brought me these cute pearly things that you put into Prosecco and I think you can find it at Lakeland Limited.
Like others have said, I would definitely bring teas and assorted drinks like squash because it's different and my goodness it is expensive here.
Enjoy your holiday in my country!