As I see it the TOCs are wary of Pre Pay for business reasons, and
given the current business structure of the railways it shouldn't
surprise anyone that they look at things from this perspective.

From: http://www.alwaystouchout.com/project/66

"The DfT recently specified that TfL's zonal fares must be rolled out to
all rail services within London by 2007, with a phased approach being
taken to achieve this with an individual train company at a time."

Once that has happened there will be much less reason for them to
resist.