If Macbook Pro Early 2011 (Running El Capitan 10.11.4) British Keyboard, can no longer make the euro symbol (€) or the acute/Irish/Gaelic fada accent (´) using the option key, then you might be facing problem.
Option + 2 now gives ™ where it used to give the euro symbol, and option + vowel now gives a circumflex accent (e.g. î) where it used to give the acute.
Though you have not done any change in setting, it might be due to update.
Heere is the SOLUTION for you :
It sounds like you may have the US keyboard enabled. Check System Preferences -> Keyboard -> Input Source.