The huge wagons you have seen with BTFS on them belong to The Bristol Television Film Services, which is the name of the company supplying the vehicles for film and television filming.
Each vehicle has a different use, one is for costumes, another is for make-up etc.
which would make sense seeing as how it's close to Bath to use the Bristol based company.
As to exactly what they are there for, I can't help you with, I'm afraid.
you will just have to be nosy and go and ask someone.
:-)