There are several bags of leaf mould FREE for allotment holders on the BCCS site – fill your wheelbarrow and help the fungi in the soil.
What is leaf mould good for? Garden Organic has an excellent page of information on leaf mould https://www.gardenorganic.org.uk/leafmould
Anecdotally I always use leaf mould on my onions – they have always grown better when mulched with leaf mould. Leaf mould has few nutrients but it is likely the moisture retention qualities that help vegetables especially during dry periods.