Jerry,
Arrival holds at LHR are normally 10-12 mins, but occasionally 35.
Once on the ground it may take anything between 3 mins (T1) and 30 minutes(T4) to taxy to your allocated stand which may or may not be vacant.
Following the arrival of a dispatcher we can park but then find ourselves at the mercy of the limited number of step drivers and wheelchair pushers.
Having finally disembarked all the pax we are free to vacate the aircraft but might have to wait for MT to take us to the next aircraft (which has also been subject to the above)
T4 can be the worst.
On a '40' minute CDG into T4 you could get 20 mins holding, land on the northerly so 20 mins to taxy, then wait 20 mins for a stand - ie one hour late on a 40 min flight.
Later on that day there is no holding, land on 27L, stand empty - ie on time or early.
We are trying. BA is trying. LHR is crowded and unpredictable.