chrisy08
If you are talking about the story published in a Sunday newspaper the government is supposedly looking to spend £500 million on rail development that would include a new station at Heathrow to bring together the Great Western main line, HS2 the high-speed line to Birmingham, Leeds and Manchester, and Crossrail, the new west-east London line.
If it ever happens it would not just benefit (if that's the right word - some say the opposite) Cardiff and Wales but the whole of the south west portion of the British mainland via the GW main line.
I'm sure the citizens of Plymouth would echo your argument for their airport which may not exist at all very soon.
TwinAisle has already pointed out the difficulties of your suggestion so I won't bother going there.