I have had very poor experiences getting any US agents to short check luggage. Very rarely successful. I have also had bags pulled early in LHR...also a bit of a nightmare and have had a couple of times when unsuccessful (and my bags head to my ticketed final destination without me). I think 1:15 is unlikely enough time to get this done.

I wish I had some suggestions. Best I can say is to do your best, try, and have some sort of back-up plan if you're unsuccessful on each account (short check/pull luggage).