Not always ...
Post from, say, the Highlands to, say, southern England is sorted latest in Inverness:-
. 1st class is taken to Inverness Airport and, about midnight, flown out to the Gatwick Mail Centre, where it's sorted and almost always delivered the following day;
. 2nd class is trucked south, most (all?) drivers go out and return in a shift, so each stage is no more than 4-1/2 hours between mail centres. 2nd class post takes 4-5 days.
. The 1st class mail plane returns from Gatwick to Inverness, the mail is sorted and delivered almost always that day, although actual delivery can be well into the afternoon in more-remote areas. Similarly, 2nd class is trucked northwards 'til it reaches Inverness for distribution.
While the two classes won't always be transported differently, the costs of transporting either class, say, across London subsidise the costs of transporting the same class to/from a relatively-remote hamlet in the south to/from a more-remote hamlet on one of the western or northern islands.