A Post Office would normally have category 'A1' planning permission, which means that the present (or future) occupiers can use it for any of the following purposes, without the need for further planning permission:
(a) for the retail sale of goods other than hot food,
(b) as a post office,
(c) for the sale of tickets or as a travel agency,
(d) for the sale of sandwiches or other cold food for consumption off the premises,
(e) for hairdressing,
(f) for the direction of funerals,
(g) for the display of goods for sale,
(h) for the hiring out of domestic or personal goods or articles,
(i) for the reception of goods to be washed, cleaned or repaired,
where the sale, display or service is to visiting members of the public.
Chris