The following series of posts have examples of the crossover between Somerset and Bristol ...
From 13th October 1980 second class mail from Taunton was being sent to Bristol to be sorted. In 1996 eight "Integrated Mail Processors" (IMPs) were installed in Bristol's new mail centre at Filton, going live from May 1997.
The initial cancels were for "BRISTOL"
or "Bristol", followed by "Bath / Bristol" (April-July 1998) and "Bath
Bristol / Taunton" (from January 1998).
In July 1997 as one of the IMPs was being introduced, it ran for a few days with "Royal Mail / office_line_1" in error.
... and here are examples of the "Royal Mail / BRISTOL" cancels.