Like other places really depends where you are. The big tourist hotspots it's very slow especially in doors. Lucky to break more than 4Mbit.
Outside of oxford street, green park, Buckingham palace etc it's much faster. Fastest I had was 52Mbit and it was consistent. 3G is also fast on three network, I've easily hit 20-30Mbit on 3G.
To give you a direct comparison I tried 3G and 4g near Tottenham court road and 4g was quicker but only marginally. 2Mbit vs around 5-6Mbit. It was a very congested area.
If I had to guess I would say it's probably not as constantly fast as places like Tokyo. I hear they have super fast communications over there and way ahead on broadband compared to us so I assume they are more stable, faster, better signal and more advanced.