Curiously, the report makes no mention of the fact that the captain apparently wasn't wearing his shoulder harnesses, and his lower harness must have been pretty loose to have allowed him to move the way he did.
Once you're well established in the climb, it's common practice to undo your shoulder harnesses. As for the lower harness, I can't speak for Captain Lancaster 'cos I don't know, but personally I tend to loosen mine a bit. Funnily enough, having worked at BHX, I always keep this incident in mind when I'm strapped in!