I've heard that all puppies are different so if she is eating enough and just sleeping a lot, she might need the rest. She's had a big trip and is probably a little out of sorts. Check her gums, make sure they are nice and pink and not white and check the skin on the back of her neck. If you pinch it up does it stay that way or go back down? If she is eating food regularly and is just sleeping a lot it might be normal. She is a puppy and I know they do sleep a lot. Of course a trip to a vet will probably ease your mind regardless.
Maybe you could take her to a different vet or perhaps there are some emergency clinics in your area?
I'm sure London will be ok
