About this time the charter offshoot called British Airtours took on the older Rolls engined ex-BOAC 707-436 and then flew them with three pilots, one being the panel operator[
Basically correct but unlike say the trident where the two co-pilots could play musical chairs On the B707 being an American built aircraft the minimum crew was 2 pilots and a F/E and this meant that one of the pilots had to obtain a F/E licence to sit in the 3rd seat.
I believe US operators have usually/always gone down the 3 pilot/panel operator route whilst UK had an F/E. This (as someone else has mentioned) was at a time when all longhaul a/c had F/E's
In the USA all multi engine aircraft above a certain weight operated by a USA airline were obliged to carry a person with a F/E licence. When with the advent of jet airliners the american pilots union got rid of the dedicated F/E and replaced him with a pilot it meant that the replacement pilot had to obtain a F/E licence.
This applied to all aircraft built up to about the late 1970s when the ruling was changed. Now during that same period British built large aircraft minimum crew was 3 pilots or 2 pilots and a F/E