Much as I take a dim view of the laws and politics of England & Wales*, those numbers include "indecent" and "obscene" messages, i.e. dick picks could be mixed up in these totals.

I suspect the actual number of un-asked-for dick picks sent each year is significantly (multiple orders of magnitude) higher than that, while also suspecting that most of those pics don't lead to arrests and what people are arrested for is in fact hate speech or threats that at first glance seem like they might be terrorist in nature, but so far as I can tell this distinction is not actually recorded in any official statistics so we just don't know.

* I left the UK in 2018 due to the overreach and incompetence shown in the Investigatory Powers Act 2016, plus the people in charge during Brexit all hating on international human rights obligations; I would've left a year sooner but for family stuff.