Mentioned several times
London is the capital of England and the United Kingdom.
In an alternate future which Piper and Phoebe visited, where Prue lived to see 2009 and was the owner of Buckland Auction House, Buckland's had four locations, the San Francisco branch as well as Paris, Tokyo and London. ("Morality Bites")
The pirates used the mystical fog to travel to London before going to San Francisco. ("Charrrmed!")