Nitrogen feed - which veggies would benefit most?

Assuming you are in the UK (perhaps you could add your approximate location to your personal profile) only beans of a variety suited to autumn sowing and hardy enough to survive the winter will grow, so runners, French beans but also most peas will not do much if anything (peas are hardier than summer beans but I recall DD's picture of overwintered pea plants which didn't stand up to a cold spell.)

Field beans are one hardy variety sown as a green manure - there is some doubt how much richer in nitrogen the soil is left after removing the plant because most it is locked up in the plant itself and not made available until it is dug in or composted.

Nitrogen is best for leafy crops.

But I agree that it really is a waste of time putting them in now- it's too cold and wet for them to germinate.  Seeds will keep a very long time if not frozen, baked or wet.  Might you use them in years to come, or have contacts who might take them off your hands?

You could always eat them