I'm sure someone will come on to correct me, and my "research" - I use the term lightly - was on a Design Council website, which has an obvious vested interest in talking up the UK - but from what I have gathered, design and the creative industries generally are a huge economic sector in the UK, it's growing, and it's set to grow further. OK, interior design is only a small part of that, but I fondly imagine (not least because my son is an art student) that excellence in design and creative fields generally will lead to prestige being conferred on those who are seen to have cut their teeth in the UK market.
Interior design definitely has a niche area where well-connected and wealthy people (often but not exclusively women) work for wealthy contacts without necessarily having any formal training. No-one who's pointed that out is wrong. It's just very far from being the whole story.