The law on agendas states:
(2)Three clear days at least before a meeting of a parish council—
(a)notice of the time and place of the intended meeting shall be fixed in some conspicuous place in the parish and, where the meeting is called by members of the council, the notice shall be signed by those members and shall specify the business proposed to be transacted at the meeting; and
[(b)a summons to attend the meeting, specifying the business proposed to be transacted at the meeting and certified by the proper officer of the council, shall be sent to every member of the council by an appropriate method.]
[(2A)In sub-paragraph (2)—
(a)“authenticated” means signed or otherwise authenticated in such manner as the proper officer thinks fit; and
So it only has to be authenticated, not necessarily by means of a signature. And it's only the version received by councillors that needs to be authenticated. The version published could have this redacted.
As for presuming consent, this is a no-no. Under GDPR consent has to be explicit and informed.