What’s the UK equivalent of US marshals?
There’s no direct equivalent that duplicates all the functions of the US Marshall’s service, although many of their duties are covered by some body or other. Bear in mind that we don’t have the equivalent of a federal courts system
Is there any particular US Marshall function you are particularly interested in comparing?
Capturing fugitives.
The police capture fugitives. Whereas in America there are Sheriff’s Offices, Police Departments, State Police, the FBI, DEA, Marshalls, ATF an verious other law enforcement agencies, Britain just has the police. Generally organised as one force per county, more or less. The police are the only ones with the power of arrest (pther than a citizen’s arrest, I suppose). There are some other groups who run intelligence and prosecution (the Serious and Organised Crime Agency, the Serious Fraud Office, Her Majesty’s Revenue and Customs, the Royal Society for the Prevention of Cruelty to Animals, MI5, and so on), but any apprehending will be done by the the local Constabulary.
That’s just handled by the police. We don’t have the division of county, state,whatever police departments. It’s split up by city or region, but there’s only one level within each*, if you see what I mean. Someone on the run will just have a notice put out by one of the police forces to all the others and also to the ports and airports.
- there are a few specialized police forces that are nationwide in scope, the largest being the British Transport Police who look after the railways, and there’s another that handles organized crime etc.
IIRC In Canada, so I assume in the UK, police or other peace officers are peace officers everywhere in the country, not just in their own little area. There is also only one system of courts for criminal offenses, as all criminal law falls under the central government.
The UK, delighting in being complicated, has three separate court systems: England and Wales being one, with Scotland and Northern Ireland each having their own arrangements.
Yes, one aspect of the U.S. Marshals Service is to capture fugitives. They are needed to go after federal fugitives, fugitives that cross state lines and to coordinate federal, state and local assets to capture the most dangerous fugitives. In countries that are not made up of semi-sovereign states with their own laws (and extradition) a separate entity is not needed.
Complicated? 3? Try 50. Plus territories. Plus federal court. Plus tribal lands. Not to mention state, county and local ordinances heard at lower court levels. And I’m sure there are weird permutations in certain states that I can’t think of right now.
Hah, I suppose I should have said “non-obvious”. Another example: The Supreme Court of The United Kingdom is the supreme court for everything in the UK, right? Well, almost, but not for Scottish criminal cases it isn’t.
But yeah, the US is on another level - makes it a bit puzzling reading some US crime novels and thrillers. 
And it’s not supreme at all - The European Court of Justice’s rulings top it [Factortame, etc.]
In areas within its competence, yes. Same with the European Court of Human Rights.
How does Scotland Yard fit in to all of this? It’s nationwide, right?
Sort of. New Scotland Yard is the headquarters of the Metropolitan Police, which is the police agency for London (except for the City of London, which has its own police). It’s mainly just for London, but the Met is so large and so old that it does have a few nationwide responsibilities, like VIP protection.
No, this is not correct. If a police force is set up under federal law, notably the Mounties, they are peace officers throughout Canada. However, if they are set up under provincial law (eg the OPP or a municipal force), they are only peace officers within that province.
No, the criminal courts are provincial, and the orders from one provincial court do not automatically function in another province, without being validated by the courts of the second province. The validation is done under federal criminal law, but it’s not one court system, but thirteen.
But other than that completely correct.
What exactly do US marshals do, anyway?
And in Westerns the town often has a marshall and a sherif. And the latter always defers to the former. Is there any reason that would be the case, or was it just dramatic license?
We do have sheriffs in Britain too, but (unlike back in the days of Robin Hood) they no longer have any law enforcement duties. In fact, they have very few duties at all, and are high most of the time.
Actually, if it raises a devolution question, the UKSC can and does hear Scottish criminal cases as it has taken over some of the Privy Council role.
As for US marshals! the new. National Crime Agency has similar powers.