In Britain, there is no Thanksgiving Day holiday, such as in the United States. Instead, there are harvest festivals and celebrations.
Until the 1950’s, turkey was a luxury in the UK. Miss Marple’s character was set in the immediate post-WWII England, so I can say with confidence that whatever harvest festival was celebrated in her village of Saint Mary Mead, did not include a turkey.
Miss Marple was never married, but she did have nephews who adored her and no doubt invited her to spend holidays with them but since a Harvest festival was not the equivalent of the US Thanksgiving holiday, she might have had a special little dinner in her own home, perhaps with a widowed friend.
Partridge would definitely be on the menu, roasted with parsnip and carrot and flavoured with sage and bacon. Any partridge left uneaten would be made into a game pie to be eaten the next day.