3 days isnt really very much time to get in shape, however some people enjoy going on a long walk/run before the flight. The fresh air is good for you, and you are nicely tired so you can hopefully get some sleep. Also, what I like to do, take a shower and puy on some fresh clothes befor leaving for an overnight flight. That way you will not feel quite so yucky wearing the same clothes all night.

Wear comfortable, loose fitting clothes and drink plenty of fluids. Also, bring plenty of reading material in case you can't sleep. There is nothing worse than being stuck with a bad movie selection and not being able to sleep. Plenty of books and magazines will keep you occupied. If you ae going to London for the first time, get some city specific literature, maybe a crime novel set in London or similar ? I always find this is a nice way to get into the mood for your trip.

Hope this helps and have a great flight, always glad to help a vet.