Can a seller ask a tenant to sign an N11 to end the tenancy voluntarily before closing?

Yes, a landlord and tenant can agree in writing to end a tenancy voluntarily using this kind of mutual agreement, and doing so avoids the stricter grounds, notice period, and compensation obligation that apply to a landlord-initiated notice such as a purchaser's or landlord's own-use notice. Because it depends on genuine mutual agreement rather than a unilateral notice, it can, in principle, offer a faster route to vacant possession than the alternative when a tenant is genuinely willing.

The key word is genuinely: this kind of agreement must actually be voluntary, and a tenant who later feels they were pressured or misled into signing can potentially challenge it, particularly if it looks like it was presented as something they had no real choice about. A seller trying to secure a quick sale should not treat this as something the tenant is expected to sign, and should be prepared for the tenant to decline, in which case the more formal notice process, with its own timing and requirements, would still be the available route. Get legal advice on how this is presented and documented if you intend to rely on it.