Hmm.... My old mate Take3 etc is more or less right, but i seem to recall that CCDS (Code/callsign distrubution system) allocates (and pairs) codes issued in the London FIR.
Hence when NAS is unavailable (shutdowns/ FLOPS) CCDS is still there so you can manually pair up code / callsign / dest / exit point.
I think?
Rgds BEX