Perhaps the council should have passed a resolution to extend the date for signing the acceptance form.
See the Electoral commission Guidance for councillors which says...
Declaration of acceptance of office
1.1 If elected, you may not act as a councillor (this includes attending and voting at meetings) until you have signed the declaration of acceptance of office.
1.2 The declaration must be made in the presence of a member of the parish/community council or of the proper officer of the parish/community council. The declaration must be made and delivered to the council before or at the first meeting of the parish or community council after your election (unless the council at that meeting permits you to do so before or at a later meeting fixed by the council). If you do not submit your declaration by this deadline, the seat will be declared vacant and a notice of casual vacancy will be published.
See also LGA Section 83