I don't really see each proposal as a replacement for a previous one, but if we're heading down that path, then Beaconsfield replaces the 1998 plans for Hedgerley, about two miles to the east. Hedgerley was a partner for Oxford, which replaces 5 other planned services. Before that the M40 ended at Wheatley, and only before then does Oxford come in to it.
There's probably a bit of showing off in there, but the point was a lot has happened here and Abbey Barns was a long time ago.
With Iver it's a bit more clear-cut as it has been on the cards pretty much the whole time from the construction of the M25 until the widening, but they varied between three sites, including one by Poyle.
I know a bit about this as I was invloved in the hugh public inquiry for the MSA's to the west of London between 2003 and 2004. Bascially planning applications were submitted for M25 Cobham, M25 Iver, M40 Beaconsfield, M40 at Hedgerley (between M25 and J2), the M4 at J8/9, and at 2 sites between J8/9 and J10. All were refused by the respective planning authorities on Green Belt grounds and at M40 J2 on local highway grounds.
As a result John Prescott decided that one planning inquiry would be held in all 7 sites and that only 2 would be approved. At the end of the inquiry , the 2 that were approved were M40 Beaconsfield and M25 Cobham.
Given the remit of the Inquiry , that 2 would be allowed and they were all refused on Green Belt grounds, the main thrust of everyones argument came on need and safety. In short, given the inspectors report was 500 plus pages the reasons for and against each were as follows.
M25 Iver - one of the most congested sections of the M25 with extensive weaving between the M4 and M25, the introduction of a services would worsen this and be detrimental to road safety.
M4 sites - No need on journeys out of London, given Heston and services at Reading, which should be ample to serve motorists on the M4. Also would not pick up a majority of M25 south to M4 west journeys(or vice verse) given the prefered route of A329(m), A322, M3.
M40 Hedgerley was only located 1 mile from J1A with M25 in a prominent position. Refused on additional weaving close to M25 junction and the fact given its prominent position would be extremeley detrimental to the Green Belt.
M40 at J2 Beaconsfield was allowed as in the Inspectors view, was that after Park Royal the A40 is a HQDC , similar to a motorway, with no online services and therefore in affect there were no services between Park Royal and Oxford a distance of over 40 miles. Given J2 was basically half away , it was offline surrounded by trees and therefore only one building was needed , lessening the impact on the Green Belt it was acceptable on Green Belt and need.
Chobham was chosen as it picked up Gatwick M23 journeys , going around the M25 towards the M3 and M4 and on this journey there was very little provision. It was accepted that Clacket Lane was only 20 miles east but the volumes of vehicles undertaking M23, M25 then M3 / M4 was high. It was also argued that with a provision here it lessened the need on the M4.
Hope that provides some insight in to why the new MSA's are where they are