A photo turned up on Reddit of a TARDIS prop in Ribble Valley in Lancashire County on September 19.
It is not known why a TARDIS was there but that is not the only account of a TARDIS in Ribble Valley.
— Hazel Dalton (@dalton_hazel) September 18, 2018
Series 11 filming wrapped on August 3, along with the 2018 Christmas special. Some filming for a Children in Need special occured on August 7. Series 12 is not thought to begin filming until November.
If a TARDIS was in Ribble Valley for filming, what that filming was for is a mystery.