well harrods is way more well known then fortnum and mason, and although the stuff is pretty expensive you should have a look at the sale stuff and maybe buy something small like a teddy bear or something. i think the sales will get more and more leading up to xmas, so a bit later on will be cheaper. marks and spencers is good for nice englishy boxes of biscuits... i dunno if thats thought to be english though in america though =S hmm.. if you go to like somewhere central londonish where all the tourists go, so like baker st, or oxford st, they have those tourist shops, where they might have small keyrings and teddy bears and stuff with london on, although im not sure if this is what you are looking for. its hard to say what you should buy, because i have know idea what is considered really english in america.. apart from tea which isnt the best present. you could buy a jumper or something from topshop because that isnt available in america and they have some nice stuff.
im sorry that this is all probably unhelpful but who are you buying stuff for?