the 91xx has different 3G bands from the 9105 sold in the UK:
UK 9105: HSDPA 850 / 1900 / 2100
US 9100: HSDPA 900 / 1700 / 2100
luckily they both have 2100 which works in the UK
on the downside it means that if/when UK operators start to use the 1900 band as well you will not have access to it.
Interesting read (for geeks) on UMTS world coverage: http://www.three-g.net/3g_umts_spectrum.pdf