Hi, it seems like you have done a lot of research for the tax ID thing, but Maybe you have one already? I live in Canada and the "Tax ID Number" we used is called the Social Insurance Number. It is issued by the government, and you don't need to actually pay taxes for having it, like if you are minor for example. I know in the USA they have a similar "Tax ID Number" that is called the Social Security Number, although I'm not sure how this one is issued and what are the criteria. All that to say, I guess you should have a similar government-issued ID in UK.... I think in the UK they call it a National Insurance Number, you might have one already, if you worked before...?
On a side note, I recently found my only son (4 years old) is on the spectrum... He seems to have a very good memory, and to be good at figuring out how things work, like mechanisms, like a door handle... Or taking a chair to access things out of reach, but his communication skills are very much lacking... as he struggles to get the motivation to learn a language... All that to say, that I found very encouraging to see what you were able to accomplish, as it gives me some hope that things will improve... I hope you won't find that offending...