Although BA did use Tristars on European flights, they were a dedicated Euro only fleet (mostly the initial aircraft which were actually ordered by BEA) rather than the intercontinental fleet filling in for extra utilisation. They had a different and much denser seating configuration. BA then moved on and did the same with two separately configured fleets of 767s.
The BA Airtours fleet at Gatwick also had further Tristars, exchanged from time to time, with an even denser charter configuration. These were fully utilised as such in the summer, but off-season were regularly chartered back to the BA scheduled European operations from Heathrow, to cover overhauls etc.