A “legal separation” is not an expression that a family lawyer recognises. Any separation between married couples, or indeed cohabiting couples, is legal. What people probably mean is, a separation that formally ends the relationship, such as the start of the two year separation period needed as a ground for divorce.
The term doesn’t really apply to cohabiting couples, because cohabitation is not legal to start with. If you want to obtain legal rights as a cohabitee, please see your solicitor about a Cohabitation Agreement.
Concentrating on the relationship that is protected by law, namely marriage, if you and your spouse are certain that your marriage is over, you may not want to start divorce proceedings straightaway. However, there may be complicated financial issues that need dealing with before waiting for a two year separation period. In this situation a Separation Deed can be drafted which has the same effect as a consent financial order in divorce, except it hasn’t been approved at Court. But it is enforceable as a contract and if need be, can be used in court proceedings to ensure the terms agreed are carried out.
Just remember, once you are separated from your spouse, whether you want to refer to it as a legal separation or not, both of you still have the same rights one against another in respect of the assets in the marriage, be they capital, income or pensions. Don’t assume that the longer the separation, the weaker the case for sharing your assets will be. Do consult a Family Law specialist if you have any doubt about this.