The National Allotment Society (NAS) is the leading national organisation upholding the interests and rights of the allotment community across the UK. 

They work with government at national and local levels, other organisations and landlords to provide, promote and preserve allotments for all. They offer support, guidance and advice to its members and those with an interest in allotment gardening. There's a plethora of useful information and resources on their website.