Perfidious Albion is a phrase that refers to the sneaky and untrustworthy behavior of England or specifically the British government. It is a phrase that has been used for many years to describe the British and their behavior towards other countries.
Let's pretend that you are playing a game with your friends. You all agree on some rules that you will follow, but then you keep changing the rules when it suits you. This would be very sneaky and mean, right? Well, this is similar to what perfidious Albion means.
In the past, England has been accused of not following the rules that they agreed to when dealing with other countries. They might have made promises but then didn't keep them or they might have taken actions that were not fair. This behavior has caused other countries to not trust England and they have called them 'Perfidious Albion.'
So, just like how it's not okay for you to change the rules in a game, it's not okay for countries to break promises or act unfairly towards others. That's why people use the phrase "Perfidious Albion" to describe England's untrustworthy behavior in the past.