Epsilon Minus:
Indeed I have considerable experience of LHR,- and of numerous airports worldwide.
If you look at a map ,- or good aerial photograph,- you can see just how close the terminals are and how walkways and travellators are indeed practical. Don't forget that the annual running costs of a single airside bus come to around £ 100k per year ( capital costs, maintainence, wages and between 2 and 3 drivers per bus), so fixed facilities come out both more practical and better. ( you dont have to wait for them). The gap between the extremeties of T3 and T2 is now a couple of hundred yards,- closer than than the further stands in T3 from the main lounge. T2 to T1 is similaly close via Queens and of course has always had a landside walkway.
The gap between the extremeties of T3 and T 5 is no greater than that between Hong Kongs main terminal and the outer group of stands,- already covered by walkways, travellators as well as a mini train,- and a number of American airports have similar underground links between concourses. Distances involved are also less than extremeties of Amsterdam and Singapore and doubtless others which provide walkways and travellators.
So..... look at the map again and the possibilities are there. LHR T1, 2,3,5 could be just one airport. It's a real and practical possibility.