Thanks for the comprehensive reply.
I was asking on behalf of someone else who does indeed have non-safeguarded facilities for NR and no TfL specific benefits. They have recently moved to Slough but still have family at Custom House to visit. They have also applied priv discount to an Oyster card.
Great stuff - they have done the "hard" part already.
They had asked at Slough station for a journey option but they staff there had no idea why no fare was available for that journey.
Yes, unfortunately it's not at all obvious to most people why you can't just buy a simple point to point fares. You'd have needed to read the bulletin that came out at the time of the core section's opening, and still remember it, to know the background.
On thinking about the options for the journey I came up with the following possibilities:
1. Buy a ticket from SLO to WDT, exit and then touch in with priv Oyster to resume the journey on the subsequent train (risky given the level of disruption on the Liz line)!
Yes, that's probably the best option if they want to maximise the discount. This would cost a total of £3.30 for a return journey, if travelling at times when Off-Peak fares apply (i.e. weekends, or on weekdays leaving Slough 09:30 or later, and touching in at West Drayton between 09:30-16:00 or after 19:00).
2. Buy a ticket from SLO to PAD but remain on the Elizabeth line to Farringdon, head upstairs to touch in with priv Oyster at the pad within the station (risky again as technically you wouldn’t have a valid ticket for part of the journey.
I would not suggest this under any circumstances. They would be travelling without a valid ticket from Paddington (PAD) to Farringdon (ZFD) - there's no "technically" about it. Overtravelling is possibly more serious than not having a ticket at all, since it demonstrates clear intent to avoid payment of the fare.
If caught without a valid ticket they would not only receive a Penalty Fare or be prosecuted, but could also lose their staff travel facilities. There might also be disciplinary implications with their employer.
Instead I would suggest exiting the barriers using the SLO to PAD ticket and touching back in at PAD if they want to do this. The fare is the same since PAD and ZFD are both in zone 1, and it avoids any risk of getting involved in a ticketing irregularity.
It's also it's an easier option, since there are only a few steps or a short escalator to the gateline, whereas at Farringdon you have to negotiate a long escalator to get to the validators.
In fact, if they're well positioned (the front of coach 2, back of coach 3, or the middle of coach 7 at PAD for quick access to the escalators) they might even manage to get back on the same train. Many Elizabeth line services have a longer 1.5 or 2 minute dwell at PAD, sometimes increased by unused engineering or performance allowances, so I've managed to touch in/out at a PAD stop and jump back on the same train.
The frequency from PAD (or indeed Hayes & Harlington/Ealing Broadway etc.) to ABW is higher than from West Drayton, since you can step back onto a Heathrow Terminal 4/5 starter. At off-peak times, that would normally mean waiting a maximum of 7 or 8 minutes.
3. Buy two tickets (as I described in my initial post).
That would be the other option, and the only one that avoid changing train (apart from using contactless at public rate), although it does cost more.
Re breaking journey with a ticket for a longer journey, I had ruled this out because of the information on this page under the Elizabeth line section dated 16th November 2022:
https://www.raildeliverygroup.com/rst/stop-press.html
There's nothing in the NRCoT which backs this up - it's merely a summary of how TfL would
like it to work, rather than the actual contractual position. That said, I can understand your acquaintance might not have the appetite for a "discussion" with the gateline staff.