Would it not have been a signaller's error for routing the train towards a block?
No for few couple of reasons.
The train has the correct headcode 9Txx for a Thameslink via London Bridge as apposeed to one routed via the three hills 9Wxx. Any alterations to the plan would need to be given to the signaller in advance.
If the train was on it’s correct route then the signaller(s) further up the line where the possession is to be taken would need to ascertain that the last train(s) have been through the section if necessary talking to the adjoining signalling centres and or control.
Based on that, the signaller controlling Norwood Jcn would not know that the line two or three control areas further up the line controlled by another signaller at the adjoining control centre has been or will closed to traffic.
Also by now there should be a fully fledged Train Management System operating on the Thameslink Core and the BML, which would give last minute adjustments to the WTT, with direct STP or VSTP inputs from control and SSMs to either TMS isolated screens giving train running advice to signallers or TMS interfaced giving the same instructions to the signalling via ARS.
STP short term planning,
VSTP very short term planning,
TMS train management system,
SSM shift signaller manager,
ARS automatic routing system,
WTT working timetable,
BML Brighton main line.