The whole gamut of Eastern European operations into the UK is interesting, as they were unusual and tended to vary from year to year. The CSA Britannia does not appear to have been scheduled into London but would make sense, as it took the week, Monday to Friday, to go to Havana and back, and would be available at the weekend when most observers at Heathrow would visit. Aircraft types in the timetable for these carriers commonly varied from what was published. The Tarom summer holiday charters from Constanta to Gatwick, Manchester, etc, used to vary the type from week to week in the season; Tarom had a handful of each for a whole range of types, Soviet, western, and (the One-Eleven) home built, and any might turn up.
Aeroflot changed to jets very early (the Tu104), but like the others when there was a large group visiting would commonly arrange a charter, for which the IL-18 was the usual choice into the 1970s. Ships' crews for a changeover were probably the most usual reason for this, which would maybe run into oddball airports like Liverpool. The expulsion of 105 Soviet diplomats from Britain in 1971 would have been a very common reason for a special flight, but in fact a passing Soviet ship was diverted to Dover and they went that way.