I love those Cadbury chocolates that come in a long stick and just melt in your mouth or taste delicious crumbled over ice cream. Also, the chocolate/orange bars are wonderful.
As for tea, nothing is better than English Breakfast Tea in London...I get the bagged Twinings version here, but it just isn't the same.
Clotted cream, scones, Harrods chocolate chip cookies -- all YUM.
If your friends have young children, Paddington Bear is a favorite. On my last visit to London, I bought my son a soft, squishy Paddington Bear that is now one of my DD's favorites.
If no children...visit Harrod's wine cellar. All sorts of great things there, as I recall, and most can't be had here. I bought my father a book on Harrod's wine selections that he still likes to read now and then (I bought it in 1991). It's unpractical, but I would really enjoy one of those tankers that look like gasoline trucks but are filled with great beer that I've see filling the tanks of the English pubs.
Overall, people just like getting gifts. Whenever I buy for someone, I always buy something I would like to receive. For the most part, that has served me well. Have fun!