Just not Vermont cheddar....unless you're trying to kill the prof.
sending something like maple syrup would be heavy as it would come in either a can or a glass jar (if it comes in any other format, as far as I'm concerned, it's not real).
For mine and my boyfriend's one year anniversary (which we were apart for) I sent cupcakes to his office with a personalized message from Lola's Cupcakes.
I probably wouldn't send your prof an American specific gift unless you were 100% sure they had never visited the states. As a study abroad prof. in London I would assume they had probably been to the US, at least once and sending something like maple syrup would be heavy as it would come in either a can or a glass jar (if it comes in any other format, as far as I'm concerned, it's not real). I would definitely check with the university (I'm assuming it's a university of London college so it should be somewhere on the main site) to confirm that they can receive gifts. I love the idea of sending your LOR's a postcard from your law school. But in the meantime, I see no reason not to send the prof something. Maybe a giftcard to Waterstones or W.H. Smith? Do you know what they like? I guess it depends how well you know them but there are loads of London companies who do interesting quirky gifts. For mine and my boyfriend's one year anniversary (which we were apart for) I sent cupcakes to his office with a personalized message from Lola's Cupcakes. If you know what they would enjoy I could recommend some places to you if you can't think of any off the top of your head.
More elaborate than a postcard from your law school, how about something cool from the school's store with the schools logo? That way the prof can have it in the office and if people ask can proudly talk about how she helped you.