Complex sentence: It acquired two new Britannias (one series 309 and one 319) of its own, which began flying the London route in place of the B.O.A.C. contract, and with which it commenced service to Beirut in 1961.
Split into simpler sentences: It acquired two new Britannias (one series 309 and one 319) of its own. Supplanting the B.O.A.C. contract, these began flying the London route, then introduced service to Beirut in 1961.