vespasia, it's in the LHR section of the TC Pt 2. Routine ops are always 'B', we'll be notified of upgrade to 'A' or downgrade to 'Z'.
Essentially the same notification is also in the LGW (10.5.4) section of the TC Pt 2 for P37, he's 'B' unless he specifically tells you otherwise. In fact the same notification is in all the approach sections of the Pt 2.
HP, thanks for the kind words
