If your tenancy agreement prohibits subletting (which most do) and you sublet without your landlord's permission, you're in breach of your tenancy terms. Your landlord could use this as a discretionary ground for possession under Section 8, and it could also affect your security deposit if the subtenant causes damage.
Subletting the whole property is treated more seriously than subletting a room. If you sublet the entire property, you're no longer living there as your main home, which means your assured tenancy may lose its statutory protections.
If you want to sublet, the correct approach is to ask your landlord in writing for consent. If your tenancy agreement says subletting is allowed with consent, the landlord can't unreasonably refuse. If it's completely prohibited, the landlord can refuse outright.
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