I have heard of the SH pay cap at LHR too. A little more information on it is that it will be applied just like the change to pay point 34 was. So congratulations your pay point (for arguments sake) 19 you continue as before. Sorry your pay point 18 and will not progress any higher until you transfer to LH.
Until it comes out in a official document go along the lines of more for the same or less and you won't be far wrong. I just hope that they tackle the big issues once and for all rather than this continual appeasement of inefficient departments