Very Urgent Question!!

What are you booking exactly, where are you doing it? Note that seat reservations on domestic UK services are for free and optional (and often not possible). 

You don't have to reserve anything for this connection. Register the train in the app or your paper pass just before travelling, Just hop on a Southern train to Shoreham, find a free seat and enjoy the ride and show the QR code or your paper pass upon inspection. 

You can't reserve seats here, this is a typical, very frequent commuter service.

I know.

The system the GWR website uses doesn’t filter out blank “counted place” reservations which are only supposed to be used in conjunction with fixed Advance fares to tie these cheaper tickets to a particular train.

They only serve to control validity of Advance tickets and quotas for these.

They do not restrict the numbers of other tickets or passenger numbers on these trains or give any priority to holders of counted place reservations over actual seats.

It is a consequence of not using public train numbers and having to bodge the reservation system to restrict train specific tickets when used on reservation free trains.

This wasn’t an issue when only journey planners selling tickets and ticket offices issued reservations, while it is irritating that GWR will issue these blank reservations I wouldn’t complain as it is so far the only way to access free reservations online for anyone who isn’t buying a ticket.

None of the train companies that primarily operate London Commuter services offer seat reservations any more along with some others.

No reservations:

C2C

South Eastern

Southern

Thameslink

Gatwick Express

Heathrow Express

Great Northern

South West Railway

Chiltern Railways

London Northwestern Railway

West Midlands Railway

Greater Anglia

Northern

Transport For Wales

Elizabeth Line

London Overground

Optional Reservations on some services:

GWR

East Midlands Railway

Scotrail

Optional Reservations on all services:

LNER

Avanti

Crosscountry

Transpennine Express

Lumo

Hull Trains

Grand Central

Compulsory Reservations:

Caledonian Sleeper

Eurostar

I think that is a full list.

The national timetable lists availability of reservations on each train and is mostly accurate, although it still shows some Greater Anglia and TfW as having reservations when that is not currently correct.

They are marked by a diamond or a R in the header.

https://www.networkrail.co.uk/running-the-railway/the-timetable/electronic-national-rail-timetable/

https://timetables.fabdigital.uk/nrt/dec2023/

Seat reservations are not possible on trains in that part of UK, just add to your pass and sit in any seats.

GWR offer them though...

Thanks for the info!

If the counted places only serve to control the number of advanced tickets, then it's even stranger that these trains are offered to book reservations on, and also that the operators (or RDG) explicitly state in their MERITS data that reservations are possible:

 

I expect the reason is that they just don’t care, the British franknstein system that avoids using train numbers works for them on their own systems but whatever way they have of flagging fake-reservation trains doesn’t translate properly to the MERTIS entries.

No doubt it could be fixed were anyone bothered enough to address it but…

The other nonsense with LNER and Avanti purposely marking trains as compulsory reservation when they are not is another level altogether. Here they are actively propagating false data for their own reasons.