Getting in is not a problem but the L1011 at least didn't get out at any useable mass. We didn't need much mass because we only went into Cambridge to ferry the aircraft to and from Brize. I suspect the 747 doesn't have the same VMCG issue as the -500.
Unfortunately the only UK L1011 performance data I have left is for the -1 so I can't back up my memory with an RTOT, maybe someone still on 216 could check it out?