Why does the M40 terminate so far outside London? A quick check of
aerial photos and a map shows that the A40 Western Avenue runs through
mostly open land between the M40 terminus and the A312 Church
Rd./Mandeville Rd. roundabout junction, with all of the junctions in
between fully grade-separated. With the M40 extended to the Church Rd.
junction, the A40 could be diverted along the path of the A4020,
running through Uxbridge and Hillingdon as far as the A312 roundabout,
where it could turn north and intersect the M40 at its new terminus.

Is there some particular reason why the M40 doesn't terminate closer in
to London? (i.e. the existing road needed to retain full access? NIMBY?
lack of money?)