The "main" London signals are 2-3kW each from an elevated site in South London. They aren't very exciting, just signals for national network stations, and most of the interesting radio activity in London and the rest of the UK is on DAB these days. With a DAB radio in London, you get all sorts of interesting formats and languages; with an FM radio, variations on CHR and AC (along with a dwindling number of pirates, mostly Black music or Turkish).
Back on topic, 648 is from a site right on the east coast of England, facing the Netherlands. As with a lot of AM, it crosses the sea path a lot better than it gets inland.
As an aside, 107.1 is from an interesting transmitter tower,
Alexandra Palace - the site of the world's first regular TV transmissions.