The issue with "no one at the gate" is false. There is, bar some freak accident, always someone at the gate to meet the flights. LX depart almost always from T2B gates now and have done since June. These gates are brand new, however, the guidance system does, on occasion fail at the last minute. There simply aren't enough marshallers at Menzies to have standing waiting at every gate for one of their aircraft to come in just in case the system fails, that's not realistic. It is unfortunate that you experienced this, but it was simply down to LHR having a broken stand and not strictly MZ. On occasion the person who marhsalls in the plane then has to run up and put the jetty on, so don't automatically assume no one is there to meet the plane, they are just prioritising what needs done. Marhsall, Chocks, Jetty, Doors.
Waiting an hour plus for bags...yup..that's Menzies fault, no argument there, that's just poor planning or lack of staff...only they can fix that and they probably won't any time soon, especially if Swissport is getting all the bad press right now they have no reason to unless it becomes spread over the news like LGW had. Airlines go to handling agents to keep costs down, and handling agents keep staff numbers as low as possible so the airlines get as cheap a price as possible.
Also, Menzies do not control the screens at LHR. At LHR stands are not allocated until the aircraft is "zoning". That is neither LX nor Menzies decision to make where the plane goes, but no doubt you would have blamed the gate staff for "opening the gate too late". It is a silly system which operates at LHR (and other UK Airports) where they say the gate will open at a certain time. If the plane is late then it will open later than that and you get people crowding round and then panicking when it goes 2 minutes beyond the "gate will open at" time. It also causes people to not pay attention and if the plane comes in 10 or 15 minutes early then that person then misses the initial gate announcement because the screen told them to come back later.
I'm not saying Menzies are perfect, they are far from it, but not everything directed at them is their fault.