If I knew I wouldn't be asking. It's just that someone had the tag
line "Always touch in and touch out" but the other day I went through
the gates at Clapham Junction and the gates were open and people were
just being herded through by the staff without the chance to stop and
touch out, hence my question.

As far as I am aware, Pre-Pay isn't valid on any national rail services
using Clapham Junction so there wouldn't be an issue there.