Originally Posted by mpallo
Is it as simple as him setting up service and us wirelessly activating him on our BES?
Yes (I do the exact same thing in reverse, BES is in UK and users in USA/CA/EU/wherever).
As long as the UK user has a BlackBerry Corporate data plan, all should be fine. o2, Vodfafone, t-mobile and orange definitely offer corp BB. I have never looked into BT, but because of their history (used to be only telco provider in UK, large history in PBX and "home" phones etc) not many people at all ever try to use BT as their mobile/cell provider.