Cambridge to Oxford return

[Crithylum]

I am considering a return journey on a Sunday (not sure which week though)
Outbound late morning, return as late as possible (arriving 01:30 Monday is fine)

(with a 16-25 railcard, searching on 2nd Feb)
The forum will sell 2x £11.30 advance singles, however these are not fast trains from Paddington.
It will also sell a £34.43 combination splitting at Southall (which I don't want)
It also sells a £11.95 + £26.90 (+£1.14) Cambridge to London (GA only) + Z1 to Oxford
It sometimes also sells a Finsbury Park split
However it does not want to sell a Cambridge to London 1-6 travelcard (£16.25 GA, £20.55 flexible) + £16.50 BZ6 to Oxford. Is this combination valid, including on all fast trains from Paddington?

 

[redreni]

Yes, that combination is valid.

There is, however, a pricing anomaly which means that the Boundary Zone 6 to Oxford off-peak day return ticket is 45p more expensive than a day return from either West Drayton or Southall to Oxford. That may well be why the forum's site is offering the split at Southall.

An off-peak day return from West Drayton to Oxford can be used in substitution for a Boundary Zone 6 to Oxford off-peak day return. It can be used on fast trains out of Paddington, because when used in conjunction with a day travelcard (which is a daily zonal ticket) it is valid without the need for the train to call at the split point. It does, however, limit you to trains that pass through West Drayton (so you couldn't travel out of Marylebone). Arguably the Southall to Oxford off-peak day return (also priced at £16.05) can also be used in this way, although strictly speaking it does fall foul of the requirement for the last station at which the first ticket is valid and the first station at which the second ticket is valid to be the same (because of the overlapping validity of both tickets between Southall and West Drayton). But in practice either of these cheaper point-to-point tickets could be used in substitution for the BZ6 ticket on direct trains from Paddington to Oxford.

Since it seems you intend to travel via Paddington rather than via Marylebone, you might as well save the 45p and buy the West Drayton to Oxford or Southall to Oxford ticket. (Edit: thinking about it, the West Drayton ticket undercutting the Boundary Zone 6 ticket may well be deliberate on GWR's part, since one assumes Chiltern would have to get some of the revenue from a Boundary Zone 6 to Oxford ticket but not for a West Drayton to Oxford ticket).

 

[Watershed]

Yes, that combination is entirely valid, including on non-stop Paddington-Reading services as one of the tickets is zonal (indeed Boundary Zone fares are explicitly intended to allow non-stop travel beyond the covered zones).

The forum's site does sell Boundary Zone fares but only if you tell it that you already have a Travelcard in the advanced settings, so you'd have to add each fare to the basket under separate searches. I suspect it's too computationally complex to offer such a split by default but happy to be corrected on the reason.