It is always surprising when a long dead topic re-surfaces! A year almost to the day!
Thanks to all who responded, but I still do not know the location of this photo. I now tend to think it was in Sussex / Brighton.
Certainly buses could be borrowed, loaned or even purloined in times of need, I've done it myself many times. This particular "loan" , if it was a loan, was long enough for the bus to be fitted out with correct blinds.
Before ser. 122 was split at T.Wells, T.Wells, Borough Green and Gravesend all had a share of the working, but I understood only T.Wells & Gravesend buses worked through to Brighton. I am not aware of any interworking between 122 & 18, but yes possible.
Ser. 18 was operated I believe by Gillingham, T. Wells and Southdown.
A thought, could there have been a temporary diversion on 18 which sent it under a low bridge and therefore low height buses were drafted in?
JohnH, your mystery TD4 could have been a replacement for a failed lowbridge at B. Green? to be replaced on the return working. I am surprised that B.Green had any normal height deckers at all, in my time all deckers were low, to give maximum flexibility.