The way the system is set up, then Thameslink cannot just implement a split service.
Network Rail have to advise them that is what is required, and it has to be agreed by the DfT too if it is published in advance!
The provision of a split service is simple, except on the Wimbledon Loop, in any event.
Every half hour between Brighton and City Thameslink, resourced by Brighton drivers, and every 15 minutes between Bedford and St.Pancras, resourced by Bedford drivers, with additionals as and when resources are available between Luton and Kentish Town.
Leaves Blackfriars drivers to work the limited Loop service, and then hoping that South Eastern can resource a service on that side.
You actually needs more drivers to run a restricted service than you do the full pre-planned service.
In any event, it won't be uploaded to the system until overnight anyway, even though Thameslink can advise out what is planned.