I’m going to go with my previous guess: around the time of the engagement and wedding, Thomas Jnr and Samantha Markle and Thomas Snr were all saying things publicly, sending letters, going on podcasts, talking to journos. They were ruining Meghan’s narrative (and we know she needs to control that at all times). They were making her look bad and at one point could have jeopardized the marriage itself - Meghan’s greatest coup of her life.
This letter was sent to the Fixated people dept because of that: it was asking the police to get them to back off Meghan/protect the RF reputation from these stalkers who were bringing Meghan and therefore Harry into disrepute. Harry and the machine behind him were the ultimate shield and whitewashing machine. The CSA allegations were in the letter as “proof” of why these people aren’t to be trusted and shouldn’t be allowed near Meghan or the RF. This is my theory, no proof at all obvs, but it makes sense why this dept of this police force in this country.
And who would have most to gain from Meghan’s marriage to Harry? Meghan, and Doria (if only as a loving mum). Perhaps one of them sent it. Perhaps Sunshine Sachs (still on the payroll at the time) sent it (they’d know who this dept is) as part of their brief to enhance Meghan’s PR image.
The point of the letter was to try to enlist state authorities in keeping the Markles out of the press, in particular Markle Snr. No wonder Meghan still has a copy, and I question who brought this to The Sun’s attention right now.