Break of journey is permitted on all portions of walk-up tickets unless the restriction code text explicitly says otherwise. I'm not aware of any GWR-set walkup fares that bar break of journey, other than the tiny handful of 8A (i.e. Anytime) SVRs which they still have.
The restrictions on a Swindon to Cardiff ticket aren't really relevant, as that isn't the ticket you hold. It's the restrictions on the ticket you hold - PAD-CDF - that matter, and more specifically the wording of the restriction code text:
The relevant part is this:
Not valid on trains timed to depart:
- London Paddington after 04:29 and before 10:10 and after 15:01 and before 19:00;
- Reading after 04:29 and before 10:30 and after 15:30 and before 19:00;
Unfortunately this leaves it a little ambiguous whether those restrictions are intended to also apply to joining such services at later stations. The use of the wording "not valid
on trains timed to depart", as opposed to "not valid
to board trains timed to depart" (which is used in other cases) would suggest that the intention is to bar the use of those trains throughout, but it's not as 100% clear-cut as restriction code C0 for instance:
Not valid on trains timed to pass through London Bridge after 04:29 or before 09:45, regardless of where you join or leave the service.
Personally speaking, I wouldn't want to rely on the TM either not checking tickets, or not trying to charge an excess if they did check your ticket and take exception.
