I have fond memories of seeing Monarch Britannias flying in to Luton during the late 1960s and early 1970s. I probably saw some Britannia Airways ones as well, but it was the yellow livery that stands out in my mind
Here are some photos from my dad’s collection. The quality isn't as good as others in this thread, but they are interesting nevertheless.
First is 4X-AGB of El Al at Farnborough in 1957:

G-ANCA also at Farnborough in the same year:

Finally, Duxford’s G-AOVT, in BOAC service, at Heathrow in 1963:

The past is a foreign country: they fly different aeroplanes there.