The agreement only needs witnessing in order to make it a deed (it already contains the necessary deed wording). You would only need the tenancy to be a deed in the following circumstances:

• the fixed term is to be greater than 3 years, or
• the signing of the tenancy is to be in advance of possession (e.g. you are signing today but the tenancy does not commence for another two weeks)

Otherwise, if you are just signing as normal and the tenant will receive the keys on the same day, there is no requirement to witness.