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.
It’s quite normal for operator to push traffic from 900 to 2100, I suppose. 2100 MHz is for capacity and 900 MHz is for coverage. It has worked with GSM, why shouldn’t it work with UMTS. However, it’s always interesting to observe how operators tune their dual-band networks. Some push traffic to 900 as much as possible, other promote better signal bars of 900. Some vendors claim that after refarming coverage of UMTS 900 is 50% better than coverage of GSM 900 due to better sensitivity of UMTS receivers. If it’s true than customers will definitely benefit from increased coverage.