How are they the ones that the NHS signpost to here?
I can't speak for trans specifically, but you'd be amazed at how easy it is to set yourself up as one of the key support/guidance points in a niche area and then get indirectly promoted via links from official bodies. And/or become the go to person for the media.
It's like a circle of constant reinforcement. Position yourself on the web as serious, established, expert mouthpiece. Person preparing "official-ish stance/lip service page" for their organisation's website skims your site. Adds a link cos they were told to find and add some helpful links. Somebody doing similar for another body sees the original link. Assumes it must be legit cos... another official-ish source linked to them. Adds the link themselves. Journalist looking for a quote sees the links on official-ish sites. Assumed legitimacy and checking out all done. Calls and turns the spokesperson into a talking head. Spokesperson does level best to keep promoting themselves as massive expert/best talking head. More links occur. Links have babies. More media calls. Baby links spawn more baby links. Establishment as Trusted Leading Expert well on the way to becoming reality. So far everybody is mosty relying on a vague "somebody else" to turn over rocks and check under floorboards (e.g. deeply dig around in their forum using history and see what they say when they think nobody is looking) for any lurking skeletons in the talking head's closet.
It's only when the shit starts to hit the fan that all the linkers and journalists start to wonder why "vague somebody else" didn't do any strenuous digging at all, back when it was just a website nobody had heard of.
8 years ago somebody explained to me how it worked, and told me she intended to use the strategy to become the national go to expert for our shared "parental choice" niche. She was a rank newbie at it. Had not actually even begun doing the niche thing when she declared her National Expert intentions.
Nonetheless she was an established media presence in two years. She talks for all of us. But in reality speaks for precious few of us. Unfortunately when the snowball has picked up speed in its rolling it is hard to stop it. By the time you realise the urgent need to make it stop before it does harm, it's too big, and just trundles massively and squishily over all your objections.
It's entirely possible that Mermaids got linked by the NHS under a similar sort of process.