From my research (and I am ready to be corrected), it would seem that you are required to do the ATPL Theory and CPL/IR in schools approved by the same national aviation authority.. .. .So you can do ATPL theory in the UK and your CPL/IR in a British CAA approved school in the US.. .. .But you can't do ATPL theory in the UK and follow it up with a CPL/IR at a Spanish or German school. (Note that BAe is a British school in Spain, not a Spanish school).