What is the ruling on the fast of one who fasted in London and broke their fast in Riyadh?

Your fast is valid, because what matters for breaking the fast is the location of the fasting person at sunset, and time differences are not considered. The basic principle is that each person is governed by the land or air they are traversing. So, if one travels west by plane, they break their fast with the sunset at their location. If they travel east, they do not break their fast until the sun sets in the location where they are present.