as I read it, it's two hours and you are entitled to compensation for a delay at either end......
as per:
'If you do take the flight
You can claim up to £520 in compensation if the delay is the airline’s fault - depending on the distance and destination of your flight, and how late it arrived. It might have been your airline's fault if there was a technical problem, or they overbooked.
You’re unlikely to get compensation if the delay was because of something outside the airline’s control, like bad weather or a security risk.
If your flight is cancelled
You have the legal right to either:
a full refund - including other flights from the airline that you won’t use in the same booking such as onward or return flights
a replacement flight to get you to your destination
If you’re part-way through a journey and you don’t want a replacement flight, you also have a right to a flight back to the airport you originally departed from.
Ask for a refund or replacement at the airport if you can. If not, you can claim from the airline later.
You also have a legal right to:
help with costs - if the cancellation delays you 2 or more hours
compensation - if you’d be delayed 2 or more hours by the replacement flight offered and you were given less than 2 weeks' notice
If you get a replacement flight
If you have to wait long enough for your replacement flight, the airline legally has to help you with things you need. This includes:
food and drink
access to phone calls and emails
accommodation if you’re delayed overnight, as well as journeys between the airport and the hotel
How long the delay has to be depends on the distance of the flight and the countries it's flying between. You can check the flight distance on the WebFlyer website.
Flight distance How long the delay has to be
Less than 1,500km 2 hours
Between 1,500km and 3,500km 3 hours
More than 3,500km 4 hours
The airline might give you vouchers to get these things at the airport. Ask someone who works for the airline if you’re not offered anything.
If they don’t give you help at the airport, keep receipts for your expenses and try to claim from the airline later. Airlines only pay for ‘reasonable’ expenses - you are unlikely to get money back for alcohol, expensive meals or luxury hotels.'