It's not apparent from the original poster's question whether travel by Britannia was on an RAF example, or on one chartered from civilian operators, for these routed in different ways, the RAF ones via various bases, the civilian ones via various commercial airports.
Most of the civilian Britannia charter flights ended with the bankruptcy of British Eagle at the end of 1968, but a few continued with Monarch into the early 1970s (using the same aircraft of course !), mainly in the winter period when their fleet was not occupied on holiday flights.