Putting aside state sponsored cyber warfare, when it comes to "routine" investigation of individuals posting criminal material (worldwide) it is usually possible for LEA to make an identification. Every day, police across Europe and further afield are successful in identifying "hidden anonymous" (criminal) posters and locations and passing the results to law enforcement in the relevant country/ies. Generally, it`s not difficult at all (though in some orchestrated dark web cases there may be extra, more sensitive, layers of investigation required) but it is time consuming, especially when other countries are involved - which is why UK based offenders are likely, from a prosecution perspective, to "surface" first. There`s also no certainty that "offenders" identified in other countries` jurisdictions will be subject to the same depth of investigation and prosecution as would be the case if they resided in the UK.
Investigation-wise, a big hurdle is often navigating the data provision bureaucracy of service providers/social media platforms, who don`t see it as a core function to assist law enforcement (which, from a business perspective is understandable). It`s worth noting that service providers and social media platforms do not receive a fee for providing the material - they have to employ people specifically to liaise with LEA, so it`s not a commercial priority. Moral obligation doesn`t seem to count for much and a massive growth in cyber crime has left the industry even further "out of pocket". Governments must make the industry more accountable, until they do , social media platforms are unlikely to follow suit with their customers.