A seller’s option to tax does not transfer to a purchaser and a landlord’s option to tax does not transfer to a tenant. Each party in the chain of supply should consider whether or not it is appropriate for them to opt to tax their interest in the property. Therefore as a tenant is sub-letting you should independently consider whether you should opt to tax the property.
Before you opt to tax, it may be worth reading some materials on HRMC website on the implications as the decision is not easily reversible. The link below also explains how to opt to tax:
https://www.gov.uk/government/publications/vat-notification-of-an-option-to-tax-land-andor-buildings-vat1614a