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I have had numerous contracts with UK landlords over the years. They are signed by the landlord and myself and they are entirely official - that's to say legally binding.
I have no idea who you would expect to 'stamp' one of these, nor for what reason.
Such contracts are private agreements between you and the landlord and are nobody else's business. Only if a dispute arises and one of us sues the other do lawyers and courts get involved, and they will rule on who broke that terms of the contract and what restitution is required.