DeepWinterSleep, you've done a great job of patiently trying to explain this, but I fear there are none so blind as those blinded by outrage.
I spend half my professional life dealing with HMRC. I can echo lots of posters and assure others that access to the PD1 line isn't the VIP fast-track perk you might think.
As to why trans people with GRCs have access to this, it is not to do with "special secrets" but everything to do with boring old data protection; legislation that is outwith HMRC's remit. It not only includes MPs and Royals and trans people (although they are the most headline-grabbing) but also encompasses tens of thousands of civil servants including HMRC's own staff, members of the armed forces, those with particularly complex tax affairs, those who are in witness or victim protection schemes and so on. It's not a particularly exclusive club.
For those who are genuinely interested in how trans people with GRCs were granted access to this, you should look up the Goodwin v UK and I v UK cases. They ran concurrently and judgement was passed down on the same day in 2002. In essence, Goodwin's NINO allowed their employer to discover they had changed sex and outed them. Letters were also sent to employers with their "old" sex referred to.
These rulings were key factors in the passing of the GRA in 2004.