In England & Wales, isolation is for 20 clear days, so much longer than scotland.
Approved isolation is important if you take pigs to shows and don't want to be on standstill for 20 days.
However we would also recommend isolation (although doesn't need to be approved) for any stock that you bring back from a show, or if you bring stock in for mating (eg sows coming in for boar hire), or for returning boars from being lent out, and if you are breeding when you introduce new stock to the herd.
Getting isolation approved used to be free in England (don't know about Wales & Scotland), as AHVLA paid the vets fee. Not sure about it now, am just getting a new ones approved, so when I do or don;t get the bvets bill, will let you all know.