Can anyone tell me whether it is possible to obtain an international transfer from the UK Deloitte office to the US? And if it is possible, how easy it is? thx

There are moves in both directions. Not transfers strictly speaking as they are separate firms, but it is broadly encouraged and opportunities do arise.

There are procedures for "transferring", although as the Admiral says, you technically need to resign from one firm and join the other.In practice, transfers out of the UK are much less common than transfers in. Deloitte (and other consulting firms) in the US generally principally staff transfer opportunities internally.