Walter Briscoe wrote:

It seems that Oyster boundaries are gradually being extended to places
which reflect transport services.
Shenfield (already announced) is a logical boundary, as is St Albans.
Services start and finish at both.

Guildford seems an arbitrary stop on the way to Portsmouth Harbour.
I have difficulty imagining Oyster going as far as Portsmouth.
Perhaps Southern Region could be recreated.

Guildford is the terminal for the stopping services via Epsom or Cobham (and
some others as well). Under the old South Western Lines set-up it was the
outer limit on the old map.