I am visiting London on Christmas Eve and have a very long stopover. I am looking for a place to go and have a good, yet not overly-priced salt beef sandwich. Preferably a place where I can also sit and chill out for some time and do some nice people-watching. Or a place where I can walk about and eventually find a nice coffee shop, or bakery/shop with nice pastries and chococlate eclairs. Any ideas or suggestions? I heard Hampstead is a good place. Any particular recommendations of places in Hampstead?