It immediately becomes obvious that the north end of the layout is flexible offering parallel movements onto and off the feeder lines with no conflicts, however at the southern end, parallel moves are not possible. Down direction Main Feeder to Down Westbury routes down Main Feeder, down Relief Feeder, Down Reading West Curve and then Down Westbury, so conflicting with any up direction routeing from the Up Westbury onto the Relief Feeder. An up loaded stone train could block the route for some time. Is it likely that a crossover between the Up Westbury and Down Westbury will be added in the future, in order that the routeing is not via the Relief Feeder?
Can I add my welcome to the forum, Mike. "Aha - here's another troublemaker", I thought; you ought to feel at home here, anyway.
On another point, the layout at Oxford Road junction is a bit odd. Trains on the Feeder Main heading for the Down Westbury will need to go right at T 2803 and use the Feeder Relief, which thus prevents parallel moves. Maybe space is a problem, but the crossover where the Up Reading West Curve line crosses the Up Westbury line would be better as a pair of points.
I think there are very few cases where such parallel moves could occur, and those only in unusual circumstances.
The original plan had the Reading Feeder Relief Line (then also misleadingly called "Down") only connecting to the Relief Lines and platforms 12-15 (which is still true), and the Reading Feeder Main Line (then also called "Up") only connecting to the Main Lines and platforms 10 and 11. The RFML is there to take trains from the Up Westbury Line into P10 and P11, and almost nothing else. You can't get into it from platforms 1-3 and 7-9, but they (apart from P9) have access to the Down Westbury directly. The RFRL is there for all the trains into and out of P12-15 via the Westbury Lines. However, through passenger trains to and from Paddington are only a minority - most start and terminate in P1 and P2. Freight trains are thus probably more important.
We have never had an "official" reason for the added connection between the RFML and P12 (note - only P12). But I think it must be this: if an up train is standing on the RFRL (at T.1728), awaiting a platform or path through one, it blocks any move onto the Westbury Line, and there is no alternative other than crossing to the Main Lines back at Kennet Bridge. Since P12 is on the Down Relief (P13 is the Down Relief Loop) it allows a "tie-breaker" move over the RFML; this is never going to be heavily used, but its capacity for up trains would still be lost for a considerable time, including margins. Of course if the RFRL is occupied, no train can be signalled to enter it via Oxford Road Junction, so the parallel move is ruled out anyway.
You can always find cases of disruption where a particular additional move adds useful flexibility; perhaps the main one here is the loss of the Festival Line, for which the Feeder Lines offer a way to turn
XC▸ trains in the Relief Line platforms. But often there are several competing examples of these - here, a crossover between the Mains on the viaduct would be one (and has other merits).
Current practice does not accord flexibility much priority - not enough to buy much by way of
S&C▸ , anyway. Parallel moves only score brownie points where the frequency of trains can't be provided otherwise, and I'm not sure even that applies to "disruption only" service patterns.