The M23 was supposed to head further into London, which is why today it starts at a very messy J7. The consultant's report for the road makes it clear that you can't plan service stations in much detail until you are sure exactly what roads are being built where, however it does make two recommendations:
The first is for a service station between Rockshaw Road and a housing estate in Merstham, which appears to be only a matter of yards before the M23/M25 interchange. Presumably, this would require modifying the junction so that the services could cater for two roads at once. One report sites "amenity grounds" as a reason to build at Clacket Lane instead.
It also suggests "a more pleasant site" in the valley between Merstham and Hooley. This could either be part of M23 J7, or on the road immediately north of this junction.
Even once extending the M23 had been ruled out, a service area here was considered as an alternative to Clacket Lane, but deemed to be too destructive with few advantages.