NHS sources are saying that the same groups people who are eligible for the seasonal flu vaccine are the ones who are at most risk of becoming seriously ill (or dying) if they contract Covid-19. [NB: There's no suggestion that the seasonal flu vaccine will protect against Covid-19. It won't. It's simply that the list of the most vulnerable people for seasonal flu is the same as the list of the most vulnerable people for Covid-19]
So the list of conditions mentioned on this page should go some way to answering your question:
https://www.nhs.uk/conditions/vaccinations/who-should-have-flu-vaccine/
(As I'm currently on chemotherapy, and taking steroid tablets, I'm obviously included in the 'at risk' category but I knew that anyway!)