The address on the caravan is Carshalton, and the telephone number matches this on the old Wallington exchange, but I don't recall ever going there in 1978, and assume that this was simply parked outside another shop selling pub mirrors, whose name appears to end in T, and probably given the large rodent at first floor level, almost certainly ...RAT. The letters before that, of which only the extreme tips are visible seem likely to be 'ICK'.
The name John Jackson is too common to be of much help in locating the building, and it isn't clear what his business might have been.
Later frames on the same film are somewhere near the British Museum and I think it most likely that this picture was taken somewhere in Camden. There aren't all that many streets in central London where the numbers go up to 338.