I'm alerting HMCR to alert them of my divorce, so I'm no longer claiming marriage tax allowance from my ex. But i need to provide them a date of when I become legally separated.

Is this necree Nisri, fianciancial consent order or decree absolute

Well, 'separated' just means living apart. That is a matter of fact. In that case it is just giving the date you stopped living together. Assuming that you are still living together and have not separated then separation will date either from the time you started living as two separate house holds or from the date of decree absolute when you cease to be married.