There are 3 Soc. in the UK that dominate the Prof ranks.. in no special order..
ISTD.. IDTA.. UKA.
Scotland and Wales have Soc. of long standing. And again, some regional ones .
In the States, there are several that may lay claim.. again.. no special order.
NADTA.. CDTA..2 of the largest and oldest ( not by english standards ) ,and several more regional ones ...The one you mentioned is the " new " kid on the block .
The ISTD gets more " press " ,primarily because it was one of the first to venture abroad , and established its self thru medal test and prof exams, building a very large network .
Prestige wise ( tho to me, they are all equal ) ISTD gets the lions share .
The " chain" school certif. are "in house", and really speak to their syllabus. Actually , their content is much more rigorous than the Intern.style ,in some respects ( more dances.. 13 as opposed to 10 )