I had terrible trouble with Roaming, I reordered a new sim and that worked. They have a newer version of their sim cards that worked. Mine was 3 years old I think. So if you're Sim is an old one I'd replace it.
I had awful trouble with roaming and 48, luckily I had a dual sim... anyway I replaced the sim card as they have some sort of new version and that worked. Sorry it probably doesn't help in London though.