"Greenham Common airbase, just to the SE of Newbury. Then put a fast (NOT 'high speed'!) rail link up to the Paddington and down to the Waterloo lines, Didcot and Basingstoke respectively."
The main line from Paddington to Exeter runs a mile north of the old runway - and via Reading you can link to Basingstoke, Didcot, Waterloo and LGW. IIRC Roskill looked at a site east of Greenham for the dreaded 3rd airport but there were issues with proximity to Aldermaston