"Considering the number of rail lines around LHR it is incredible they took so long to build the Heathrow express line and even then failed to connect it to the Westward part of the GWR line whereby it could have gone on to Reading which must have the best connectivity to anywhere in UK"
But remember by the time LHR started to really expand we were into Beeching era - railways were doomed........... everyone would go by car - hence the M4 right next to the airport. The idea of traveling by Tube to Heathrow was considered to be bizarre - no glamour at all .
Not building the link to Reading & the West was criminal - and selling the land off even more so - but it was probably to protect the vast profits being made by the Heathrow Express. Remember that the GWR stopping service wasn't even allowed to put "Heathrow" on the destination boards at Paddington to start with - they were officially only to W Drayton and then ran into the central area as a sort of hidden service to protect the revenue stream