It is very important to distinguish between actual surrender and an agreement to surrender:
24(2) "The last foregoing section shall not prevent the coming to an end of a tenancy by surrender."
38(1) "Any agreement relating to a tenancy to which this Part of this Act applies shall be void in so far as it purports to preclude the tenant from making an application or request under this Part of this Act or provides for the termination or surrender of the tenancy in the event of his making such an application or request".
Lease of a pub - 3years from 1/12/92
Aug 94 T asked the name of his partner to be added to the tenancy new lease for same period granted with name added
Contractual term expired tenancy continued under s24
L wrote offering a new temporary tenancy lower rent tenancy at will
T accepted offer in Feb 96
Arrears of rent
Oct 96 L terminated tenancy at will
Possession proceedings summary judgment sought
L said that continuation tenancy had been surrendered when T entered into the tenancy at will.
A joint tenancy cannot be surrendered by the action of one tenant (Hounslow LBC v Pilling  1 WLR 1242)
Void under s38 [There were other arguments that are not relevant]
The tenancy at will was plainly intended by Gibbs Mew and Mr. Gemmell to replace the tenancy arising out of the 1994 lease with which it was inconsistent
T was estopped from denying that he had surrendered the tenancy.
His partner had played no part in the proceedings was not in occupation as between T and L he could not deny that the tenancy at will replaced the continuation tenancy.
Section 38(1) does not preclude an actual surrender.
Note that a straightforward agreement to surrender a tenancy at some future date is also void because it purports to preclude i.e. it has the effect of precluding the tenant from making an application under the Act (Joseph v Joseph  Ch 78).