I suppose you could use the argument that your not recieving instruction for a UK rating so therefore you are PIC legally. The other guy is just a safety pilot.
Its not really the UK authorities you need to worry about its the FAA. And if they will accept it for the IR rating. Another method I have seen UK pilots use is run a FAA and JAR log book. I was told to do that whenever I go over to the states. Certain US FI's have a nasty habit of writing all sorts of !!!!e in your log book which they say they are legally allowed to do. Which one tosser tried to do to me. He was most upset when the log book got left in the bucket when i left and when informed that my proper log book was in the UK, went in a right sulk.
So log it all in both but with a change in PIC name then ditch the FAA one after you have finished the course.
You never know the FAA allows all sorts of weird and wonderfull ways of logging time.
I wouldn't bother asking the CAA just the FAA. The CAA if in doudt will always say no to anything to do with FAA licenses.
MJ