When the M25 was reaching its final stages of completion, it was decided four services were required: Thurrock, Clacket Lane, South Mimms and finally one at Iver, just north of the M4 junction. Iver appears to have picked up the tab as Colnbrook was too difficult to build.
The original plans were discussed at a public inquiry and would have been connected to Iver railway station to create a park and ride site. This never materialised, and in 1998 the services were the subject of another public inquiry.
It should be noted that had the services gone ahead they would have been very close to the M4 junction. It looks like the reason why the plans were dropped the first time was that plans to widen the M25 took over and would have got in the way of any plans for services here. It was also acknowledged that the environment would have required a challenging layout, probably with one amenity building and flyovers for traffic on the other side.
Iver was itself an alternative. Services have previously been considered to the south of the M4 as far back as 1977, in the Colnbrook area, but it was ruled out and attention moved to Iver.