Moving from the American system into the British system is probably one of the trickiest transitions of all, and success very much depends on the age of the student and on his/her ability and willingness to adapt and take on the extra work that will undoubtedly be required. It would be exactly like jumping onto a jogging machine already going at full tilt.
We are moving back to the UK this June, once the children's school year is completed. My middle son will have completed his first year of the two year IB course. I approached the nearest IB school to us in terms of commutable distance, but they suggest that he start the IB all over again.