So, apparently in the mystery letter sent by Roslyn Loveless to the Met she writes:
“I questioned Thomas Snr — he denied accusation but stated that he went to the Philippines and Thailand because 12-year-olds are legal there.”
The accusation she questioned him about isn't given in the media reports yet, but it sounds like she heard some accusation and asked him about it, rather than having any first-hand evidence of her own. He denied it but then, for some reason, allegedly said why he went to the Philippines and Thailand. I don't know about Thailand but the age of consent in the Philippines was 12 until a few years ago when it was increased to 16.
If this is indeed what the letter contains, then there may have been nothing for the Met to investigate or pass on to another force abroad because it's very possible that no law was broken. Even if that wasn't the case, the allegations in the letter are so unspecific it would be difficult to know what to investigate. Moreover, Loveless could have been referring to allegations from the late 1960s and early 1970s (when she was married to Markle), which would have been outside the statute of limitations.
Also, we don't know just what the rest of the letter says or what the accusation against Markle Snr that Loveless refers to is or even when this conversation took place. Markle Snr and Loveless divorced in 1975, which is 50 years ago now, 44 years before she sent the letter. The conversation could still have been some years later, but even then it sounds like they were having a row. If he did indeed say that then he could have just been saying whatever nasty things he could think of in the heat of the moment.
Personally, if my dad's ex had said that about him then rather than immediately cut him off, I would confront him with these accusations, and I'd also bear in mind that my dad's ex is a problematic person herself.
Not defending Thomas Snr if he did do that stuff, even if was legal, but this letter doesn't quite chime with the timeline and narrative Meghan is trying to shape.
https://www.thesun.co.uk/news/40806546/thomas-markle-ex-wife-sick-confession/