Tom Barry wrote:

The example I'd toss out would be Chiswick-Brentford (SWT), Chiswick
Park-Boston Manor (Tube) and Acton Main Line-Hanwell (FGW), all Z3-Z4
trips over broadly similar distances (2, 4 and 3 stops respectively).
I know the first one is £2.10, the second is £1.10 and I'm pretty
sure the third is £1.10 as well, although I'm open to correction. Can
anyone confirm; are the existing National Rail PAYG schemes all
on the Tube fare scale even when you aren't going between Tube
stations?

In fact NREs shows exactly the same fares for the SWT and FGW examples.
Which is as you'd expect, as although the FGW journey has PAYG it is not a
dual availability LU/NR route, ie the cash single isn't £4.00 like the
Finsbury Park - Kings Cross BBC example...

Paul S