When I was in the Royal Navy we used to wear poppies from the first of November as per the uniform regulations in BR3 (1), these are the Royal Navy regulations taken from BR3(1)
aa. Poppies. Poppies may be worn to commemorate the National period of
Remembrance from the time they become available until the day after Remembrance
(Armistice) Day. Only official Royal British Legion poppies are to be worn in uniform.
Poppies are not worn by those on parade at the National Cenotaph Parade in London.
I am am sure the Army and RAF will have similar BR’s and wording.
I think the Royal British Legion has started selling them earlier over the past decade or so to increase sales.
Hope this helps.