I had only little time when I was recently in London to check O2's UMTS 900 coverage in London but I've been able to catch a few things nevertheless. When taking the train from outside the city to Paddington station I noticed that the UMTS 900 coverage already started in the outskirts of London, well away of the city center. Interesting data point.
While roaming on foot in the city center I was on both the 900 and 2100 Mhz band outside in the streets. When moving further indoors I was sooner or later always falling onto the 900 Mhz layer, as expected, but not before the last little bit of 2100 MHz coverage was gone. In the other direction my mobile jumped back to the 2100 MHz layer as soon as it was found again. In other words, the 2100 MHz layer seems to be preferred at all costs while the 900 MHz carrier is only used in places where 2100 MHz coverage is lost. Not sure if the same applies while the UE is in DCH, however, as all tests were performed in idle. Something to be investigated next time.