On 17/12/2020 14:41, Peter Johnson wrote:
I have been trawling digitised newspapers for travelling post office
stories

I had a friend (RIP) who bought stamps at one minute past midnight
on the day of issue from the all-night post office (Trafalgar
Square?) and then rushed to catch the travelling post offices
leaving London in order to get the very special "AM" cancellation
because the TPOs franked mail with the date of leaving London
otherwise you'd get a first day cover postmarked the day before!