He started fasting in London and broke his fast in Riyadh

Your fast is valid, because with regard to breaking the fast, what matters is the place where the fasting person is when the sun sets; the time difference does not matter, whether the day is long or short. 

In Fataawa al-Lajnah al-Daa’imah (10/296) it says: 

The scholars are unanimously agreed that the fast lasts from dawn until sunset, and each person who fasts is subject to the rulings on the place where he is, whether he is on the ground or in a plane that is in the air. 

In another fatwa (10/295): 

The basic principle is that with regard to starting to fast, breaking the fast and the time of prayer, each person is subject to the ruling of the land he is in or the air through which he is travelling. If the plane he is in takes off a few minutes before sunset and the day continues for him, it is not permissible for him to break his fast or to pray Maghrib until the sun sets from the point of view of the air through which he is travelling, even if he passes through the airspace of a city whose people have broken the fast and prayed Maghrib, so long as he can see the sun. 

Based on this, whoever starts to fast then travels in a plane towards the west should break his fast when the sun sets in the place where he is. Similarly if he travels by plane towards the east and wants to fast, he should not break his fast until the sun sets in the place where he is. The time difference does not matter. 

And Allaah knows best.