The clue is cunningly hidden in the wording of section 172 of the Road Traffic Act 1988.
What is slightly less obvious is that "beginning with" is an exception to the legal principle that the date of service is day 0. In this case the date of service is day 1, so it is 27 days *from* date of service.
N.B. Service by first class post (for such documents) is when it is delivered (to the addressee's last known address), which is deemed to be 2 working days after date of posting, unless the contrary is proven.