I think this is the correct defination (and if it isnt, I am sure I will be corrected!)
To be separated, all you have to do is live apart. There are many couples who decide not to divorce but you should carefully consider your options with a solicitor. It is commonplace to make a Deed of Separation to record any agreement regarding financial matters, children and plans to divorce or not. Care needs to be taken before signing any document as this could be relied upon if a divorce takes place at a later date. If you have both received legal advice, the court would prefer not to overturn the agreement at a later date provided that you had both been honest and there has not been any change in your circumstances.
When you are living apart, you are classed as separated by the Inland Revenue and by the Benefits Agency.
I would have answered your question by saying that ' legally separated ' means, either that you have a decree of judicial separation, freeing the parties from their obligation to cohabit, or a Separation Agreement whereby the parties formally agree to live apart.
Either will prevent you from relying on desertion as a ground for any subsequent divorce.
It is even possible to be separated if you are living separate lives under the same roof, although there are evidential difficulties in proving this.