I think Shuttle Diverts were designated in case of SRB failure soon after liftoff, hence the one in Spain; another was Banjul in the Gambia and I believe NASA or the US Govt funded its modification eg parking and infrastructure. The runway was already quite long, and with low traffic it was ideal as the trajectory to inject into orbit from Canaveral was close by.
All this means that in spite of rumours, nowhere in the UK was suitable.