What do you think the result of that would be?
I wouldn't know, I've never been in the position of having to ask.
A typical clause in an Assured Shorthold Tenancy is:
The Tenant can request to sublet part of the Property for either the whole or part of the duration of the Tenancy. The Tenant must not sublet any part of the Property without the consent of the Landlord in writing. Such consent must not be unreasonably withheld.
If a landlord did refuse their decision would have to pass a reasonableness test.