UK versus US expiration dates on diet sodas
First of all, sweeteners like Aspartame lose their sweetening property over time, as you have noticed yourself. This is very strongly tied to the pH level they are in.

US and UK/EU coke (or any country) is different, but you usually only notice the difference if you have them available for direct comparison. Coca Cola issues licenses and the plant managers produce the coke, but they do have a certain bandwidth they can operate in - usually to cater to local issues, like component availability, price, water quality. You can even taste differences in different batches of the same production plant, if you have them available for direct comparison and if you have a trained taste.

So, the deviation of the pH level in the different products results in different best before dates. The US coke pH level deviates more from the optimal level for Aspartame than the UK coke, so the US coke has a shorter best before date.

Whether the different pH level is on purpose or a necessity: no idea.