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Whilst the clause could be enforceable in theory you could rely on the provisions of the Competition Act and the Unfair Contract Terms Act.
This is because there should be free and open competition.
To stop you from sourcing a product elsewhere is against competition
If they try and enforce this you could report it to the Office of Fair Trading and/or Trading Standards
So whilst it is not strictly illegal it does prevent competition and you could challenge it on that basis
Can I clarify anything for you about this today please?