Interesting one certainly.
Having re-read the legislation it would seem that the Order is in effect from the moment it is made. That is to say once it has received whatever delegated authority it needs to become a legal document (i.e., it is signed and sealed by the right bod at the LPA). See section 4 of the catchily titled Town and Country Planning (Tree Preservation)(England) Regulations 2012 [my emphasis in bold]:
"4.—(1) An order shall not take effect (other than provisionally in accordance with paragraph (2)) unless and until confirmed by the authority, and must be confirmed no later than the expiration of the period of six months beginning with the date on which it was made.
(2) Until confirmation an order shall take effect provisionally on the date on which it is made until—
(a)the expiration of a period of six months beginning with the date on which the order was made;
(b)the date on which the order is confirmed; or
©the date on which the authority decide not to confirm the order,
whichever first occurs."
Then note the following provision of the same [again with my emphasis in bold]:
"5.—(1) As soon as practicable after making an order, and before confirming it, the authority which made it shall—
(a)serve on the persons interested in the land affected by the order—
(i)a copy of the order; and
(ii)a notice containing the particulars specified in paragraph (2);
(b)make a copy of the order available for public inspection, in accordance with paragraph (3); and
©in the case of an order made following service of a notice under section 211(3) (preservation of trees in conservation areas), serve on the person who served that notice the information specified in sub-paragraph (a)."
That says to me that serving is a secondary thing; it is irrelevant to the pure legal existence of the Order. The Order comes into effect as soon as it is made (provisionally of course) and the LPA must serve it as soon as they can. It can't come into effect on serving because it would exist in a strange half reality after the postman had started delivering at one property and until the last party had been delivered to.
That said, not being served or not being told that an Order exists (either over the phone or via a website would certainly be decent defence. Nailing someone to the cross for an offence committed in the hinterland between the lid going back on the pen at the council office and the copy arriving in the hands of the interested parties would probably be contrary to natural justice.