As the subject line above says, I wish to downgrade my account. I've looked in Manage Account, and the only discriminating option there is a link to renew the Family Membership.
As Xbox live in the UK is under UK law, you are legally obliged to offer the service to downgrade. Moreover; Trading Standards state that you are not obliged to sell a subscription upgrade without offering a subscription downgrade within the same sales area. (Sales area being your account management, payment options and renewal screen.)
So please offer me the link that Xbox live is legally obliged to display! If it has to be done by telephone, email or letter, then Microsoft ARE breaking the law (again).