On 10 Jan, 09:43, John B wrote:
because there is no point on the journey when he doesn't
have a valid ticket.

I've never used Megatrain, but AIUI you don't ever get issued with a
ticket. The staff at Portsmouth will tick you off a passenger list
when you arrive (or rather, won't). What you need to know is whether
that list is referred to by ticket staff later, either on the train or
at Waterloo.

U