Not everything in your tenancy agreement is enforceable. Under the Consumer Rights Act 2015 (or the Unfair Terms in Consumer Contracts Regulations 1999 if your tenancy began before October 1st, 2015), a term is considered unfair if it creates a significant imbalance between your rights and the landlord's rights, to your disadvantage, and it wasn't individually negotiated.
Common examples of potentially unfair terms include requirements to use a specific cleaning company at the end of the tenancy, clauses that charge you for professional cleaning regardless of the condition you leave the property in, excessive penalties for minor breaches, restrictions on having guests stay overnight, and clauses that allow the landlord to enter the property without reasonable notice (a breach of your right to quiet enjoyment).
If a term is found to be unfair, it's not binding on you, but the rest of the tenancy agreement continues to apply.
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