Proof of onward travel by flight from another country

Let's say country X requires proof of onward travel to permit my entry. If I had a flight out of country X within the duration of my maximum stay, I have no problem. But what if I have booked a flight that originates in a different country, country Y, before the duration of my maximum stay is exceeded?

In order to catch that flight from country Y, I will have to have left country X on time. Does this flight from country Y count as proof of onward travel?

I am aware that there may be differences between countries and that it may depend on which country I am from. Further, it could depend on other factors such as the locations of countries X and Y.

In my case, I am from the UK and specifically considering Thailand, Indonesia, Malaysia, Singapore, Brunei, and the Phillipines. However, I am also more generally interested in whether there are any generalisations that can be made.