This is a closeup of one out of three mastcap antennas fitted aboard Britannia. These were actually type AJE aerial outfits which gave Britannia UHF capability. Technical specifications for the AJE aerial outfit can be found here.
CREDITS or REFERENCES
1) Graphic is a screen capture from a Britannia documentary, US Public Broadcasting System
2) Mast Cap Aerials; Clive Kidd, Collingwood Heritage Collection. <cjckidd(at)waitrose.com>Septa 29/18