So this service has an issue. There's a 8-car consist on what I believe is platform 6 blocking the switch you're routed on giving you a neverending red and blocking a Brighton bound train as well.
Yeah this happens on a couple of services. One workaround is to spawn on foot a few minutes before your train is due to depart and jump in the cab of the train that is fouling the points, then drive it up to the buffers. This means that you can then set up your train and start the service on time.
Oddly, this problem only occurs if you choose the service from timetable mode. The offending train arrives in platform 6 at 10.49. If you spawn there earlier and watch it arrive, it pulls right up to the buffers, clears the switch, and the signal for the Lewes train turns green straightway.