All ACC's have to monitor 121.5MHz as it is an ICAO requirement. For both London and Scottish FIR's this is done on NATS behalf by London and Scottish Mil. Both civil centres have the capability to select 121.5 MHz when required.
The position fixing bit is a requirement of the CAA in the licence issued to NERL. It has to be on VHF (not necessarily 121.5MHz) and again is delegated to the Military operation. Although I don't know for certain it seems likely that both of these services are covered in the NATS/MoD contract.
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