Using American licences in the UK:
An FAA PPL is valid.
An FAA CPL is valid, but only for limited purposes and only with permission from the UK Department for Transport.
An FAA Instructors Rating is valid, but only for limited purposes and only with permission from the UK Department for Transport.
eg An owner of an American registered aircraft may apply to the DfT for dispensation for an FAA Instructor to give him instruction in the aircraft and/or for an FAA CPL to fly the aircraft for the owner.
NB: This is only a summary of the general principles, not an exhaustive list.
Training / Job prospects:
You'll find a great deal of helpful information on training and job prospects on this thread -
Link
Tudor Owen