BURNLEY, England—Tony Bennett took Brexit preparations into his own hands at dusk one Friday this month, defying authority to reinstate a venerable element of British culture.

All it took was glue and stick-on numerals.

The retired lawyer crept through a park, he said, and up to a footpath sign. It read “1.5km.”

Unacceptable!

He took a numeral and pasted it on to make the sign read, simply, “1.” That’s one mile, one glorious British imperial mile, rounding up a bit. No need to add an “m,” Mr. Bennett said, because Britons would know what it meant.