Mmmmmm… how sure are you that it is Winston Churchill ? I can see the resemblance with the gent at the end of the sitters, but I do not think it is Winston…
The ship is clearly a large[-ish] one – being able to fit a ceremonial guard, plus rows of seated guests and the large stanchions would suggest a sizeable quarterdeck. This would indicate that the type of ship is a cruiser. It is also clearly post-WW2 = all those rows of medals being worn show that
The New Zealand connection offers a clue – the Royal New Zealand Navy commissioned three cruisers post-WW2:
BLACK PRINCE – 1946 – 61, but laid up in reserve from 1955
BELLONA – 1946 - 56
ROYALIST 1956 – 66
SO, if this is a photo showing RNZN personnel… on a cruiser… it must be one of these.
Could I suggest the most likely would be the ROYALIST. She was taken over by NZ in 1956 and checking with contemporary newspapers, it was reported that the hand over, at Plymouth [UK], in July 1956, was attended by the NZ Prime Minister and his wife [Mr & Mrs Holland I believe}, who was accompanied by the First Sea Lord, Viscount Cilcennin
Could these be the people in the front row seats ?
For a history of Royalist in NZ service, see:
https://rnzncomms.org/what-where-wither-when-hmnzs-royalist-1956-1966/...and you can see the extent of the quarterdeck of this class of cruiser in this pic:
https://preview.redd.it/1czrxws0le281.png?width=960&crop=smart&auto=webp&s=885f0c6294c759ce9304c869ff12ce42496ea665