While 'any ATP system' will indeed provide a trainstop function, will
Network Rail fit their signalling systems with another form of train
protection to enable compatibility with S Stock? I doubt it.

What will probably happen is either the new trains will have both
tripcocks ATP, with the NR signalling remaining as it is at present, or
NR will insist the new trains meet their standards and be dual fitted
with Westinghouse ATP antennae and TPWS.