The latest form of UK passport requires the holder to sign, unless the holder is aged 11 or under or is unable to sign because of a disability (in each case the passport will state that no signature is required). The passport will not be valid for travel until it is signed. Instructions as to signing should be received with your new passport. If your passport is being used as identity for notarial work, it must be signed.
For further information please see: https://www.gov.uk/government/publications/signing-your-new-passport/signing-your-new-passport