While in London recently, I had my network analyzer with me to see who of the UMTS network operators uses how many 3G carriers (frequencies) in London.
Back in February at the Mobile World Congress in Barcelona, I noticed that the nework operators spared no expense on the exhibiton ground and had fine grained coverage installed in the different halls and used most of the carriers available in the UMTS band. In Barcelona itself, however, only a single carrier was used by each.
Back to London: Here, the situation is similar: Despite the recent uptake of 3G dongles, all but one operator use only a single 5 MHz carrier so far. The exception is Vodafone, who's consistently using 2 carriers throughout London, even in the outskirts. In some areas, they might even use three, as a third carrier is configured but I didn't pick up a signal on that one anywhere.
Interesting facts and good news, there is still enough space on the 3G band for further capacity enhancements when they become neccessary.
Kudos to Vodafone for their 3G network capacity in London, it's unmatched at the moment!