That's permissive signals, not shunt signals. It's a Calling on or Warner route, not a Shunt route, therefore it will be fitted with facing point locks and the overlap for the signal with the permissive aspect will be sufficient to protect the occupied section (in combination with AWS/TPWS/Approach control).
With regards the OP's question, if the route has facing point locks (almost all electronically operated points have facing point locks by default), then the Operations Manager can write an instruction to permit their use. These can be one-off, issued direct to the Signal Box or in the Sectional Appendix. If there's a conflicting overlap then the instruction will include keeping the signal in rear at danger (double-red). Obviously, if that signal is not controlled by the box then the instruction cannot be issued (or you would need a handsignaller to key the signal back to danger for every move). This would be why some locations allow the practice and others do not.