What's inexpensive that I can buy in London that I can't buy in America?

Note about Marmite: speaking as someone who was just in the UK not too long ago and didn't check her luggage, jars of Marmite are confiscated at security as they are considered a liquid. They'll tell you you can buy in the terminal, but they lie. Oh, how they lie.

http://en.wikipedia.org/wiki/Gentleman's_Relish

UK-to-US: Mature Farmhouse Cheddar cheese! No kidding. "Sharp" American cheddar almost always isn't strong enough compared to a decent English cheddar. I miss Wensleydale, too. Add a couple of jars of Branston pickle to go with it; maybe some pickled onions too. (Branston is available in the US but generally only in the smallest size; Marmite likewise, and if you like Marmite you probably want the big-ass jar.) Hob-Nobs. Penguins. US-to-UK: there have been a number of similar AskMes in the fairly recent past; "expat" might be a good search to start from?

I'm from New England, so my gift of choice for foreigners is always pure maple syrup. Grade B is the best.

Cadbury. Theirs is a vastly superior version vs. the U.S. (and, very difficult to find here in America).

McVities, Hob Nobs, Twinings, and a basic line of UK-made Cadbury are all easily purchased in the US at places like Cost Plus World Market. Unless you're a serious junkie, I doubt the cost savings would be worth the hassle.

For you: http://www.amazon.com/exec/obidos/ASIN/B000F9X40O/metafilter-20/ref=nosim/! Also, as trunk muffins said above, Twinings is good stuff. If you can make it to their http://twinings.co.uk/our-stores/twinings,-216,-strand,-london, you should (if you like tea).

If you are leery of taking maple syrup (Grade B is indeed the best), the http://www.vermontcountrystore.com has maple sugar powder.

Fortnum and Mason is not very close at all to Stratford Westfield. What is, though, is Spitalsfield Market - you can take the Central Line to Liverpool St and walk from there. There's interesting bits and pieces there if you aren't that interested in chains. If the Boots in that Westfield is anything like the one in W12, it will carry a lot of different ranges such as Liz Earle which you might not get there, and also their Christmas gift selection is out so if you like, say, Soap and Glory, you can pick up something interesting. This isn't ideal for you with regard to hand luggage, but the own brand bubble baths and shower gels from Waitrose are really nice. I have the Ginger and Clementine one in my bathroom right now. Last year they did mince pies that came with pine-scented sugar, so it might be worth looking at their Christmas offerings (mince pies have a long shelf-life). Christmas pudding I think is pretty rare in the US! Also, I hear Americans are pretty astounded by the variety of crisp flavours we have. Kettle Chips, the posh Walkers Sensations range, and Tyrell's do interesting flavours: the Walkers lamb crisps are spookily identical to the real thing. Paperchase do some nice stationery bits and pieces, as does John Lewis. In terms of foodie presents - Microplane graters aren't that easy to get over here, unless you want to pay a lot more. I personally would want a lot of weird junk food for the novelty value! I asked a colleague to bring me some popcorn seasoning back when he went over there - flavoured savoury popcorn is a big thing here. Stuff like the Martha Stewart magazine might be interesting.