Thanks for this update - it looks good and I think it's a bit clearer to me (I'm more of a visual person).

I've got an FCO-LHR-SIN booked later this year, and flying down to Rome the night before may prove hard timing wise, so considering doing a back-to-back.

If so, would plan to drop luggage at LHR for BA11 and fly HLO from LHR-FCO and back; but I'm not too sure what the comments actually mean?

FCO: Three orange smileys
Back to back: Reasonable (return shuttle to main terminal); Transfer: Reasonable; Landside: Reasonable

I flew to and from FCO a few months ago and if I'm honest I don't remember much about getting to/from the plane so not sure if my potential plan is foolish or not.