I'm sorry to hear that growstuff. It's awful that your GP's dismissive attitude affected the outcome.
I see here another example of pigeon-holing you as a slightly neurotic patient, which obviously you were not. They do sound negligent.
(As you ask, the only gp referral letters I've read are one or two introductory liners because I'm lucky to have private medical insurance and very often - certainly before covid - I am seen before the letter is even typed, however insignificant the problem).
We don't know what was in the gp's referral letter for this young boy. I do hope all his issues associated with hypermobilty were mentioned. DGS has crippling pains in his legs, like growing pains, but it can affect his tummy too, which then increases his anxiety, which in turn affects his education. So, the illness in the Kew boy could affect more than just the one joint used to hold a pencil. Something called Ehlers–Danlos syndrome would also need to be considered by a consultant.
I just hate to see anyone dismissed, disbelieved, or treated differently for whatever reason. It's wrong. And yes, I know, on occasions it's the law, but I too am glad it has been highlighted.