Regulation 261/2004 is a European Union law, which is still part of UK law, that protects passengers in the event that their flight is:
Cancelled, or
Delayed, or
They are denied boarding – ‘bumped’.
It applies if:
You are taking a flight with any airline which departs from any EU airport, or
Your flight is on an EU licensed airline that departs from an airport outside the EU and is returning to the EU.
So all British Airways, Virgin Atlantic, Lufthansa, easyjet, Ryanair, Alitalia, TAP flights and many many more airlines would be covered whether they were flying in or out of the EU because they are all EU licensed airlines.
However if you were flying on say Emirates or Cathay Pacific or American Airlines or any other non-EU licensed airline you would only be protected for the outbound flight not the inbound flight from outside the EU.
You must arrive at check-in by the time stipulated in writing by your airline, tour operator or travel agent or, if no time is stipulated, at least 45 minutes before departure.
You are covered if you have a ticket as a member of a frequent flyer programme but not if you are travelling free of charge e.g. an infant.
It applies to ‘operating air carriers’ i.e. the airline on which you flew – not necessarily the airline that sold you a ticket. This can cause a problem with code-sharing flights where airline A, an EU licensed airline, sells you the ticket but the flight is on airline B, which is the ‘operating carrier’, and is licensed outside the EU. You will be covered on the outbound flight but not the inbound flight so check your ticket carefully.
For the moment EU 261 still applies in the UK despite Brexit although
minor amendments have been made to bring it into line with UK law such as changing the currency from Euros into pounds but your rights remain essentially the same.