No 28 days.
But thats for action to start. The announcement must be 7 days before that.
233 Calling of industrial action with support of ballot
(b)there must be a call for industrial action by a specified person, and industrial action to which it relates must begin, before the ballot ceases to be effective in accordance with section 234..
234 Period after which ballot ceases to be effective
Subject to the following provisions, a ballot ceases to be effective for the purposes of section 233(3)(b) in relation to industrial action by members of a trade union at the end of the period, beginning with the date of the ballot—.
(a)of four weeks, or.
(b)of such longer duration not exceeding eight weeks as is agreed between the union and the members’ employer.]