oh hello! I think it depends what you are needing it for , and for how long. Is it because you cannot get good manure, or is it to protect or help improve the soil? I grow green manure on my allotment in some areas. I use mine for protecting the soil from loosing nutrients over winter. There are quite a few. I am thinking of Phacelia this year. I used mustard last year. Something that isn't too hard to dig in come spring is important to me this site is full of useful info on when , why and what to sow green manure
My Dad uses overwintering broad beans, as he always has a bumper crop and has some seed over to dry. Some of them he lefts mature for a crop, others he gets rid of before they flower, using them just as a manure. I might give this a bit of a go this year!