The reasons for Northolt not being an option are in other threads and the Search function will help you find them. One of the key points is: You would then be closely overflying a suburb of London that has long had a low number of flights, of small a/c and only during daylight hours - unless an emergency. Try upsetting another suburb full of rich voters?
Secondly, the costs of building a rapid transit across 100% developed land (leave alone under it)?