If they were legally married then she was his wife. By British law she was not a possession but if this had been abolished by his dictat (he was ruling the world at the time) then he could decalre her to be whatever he wanted to be.
Legally thats true. But as far as how the Master saw her she was a play thing a toy to be used when it suited him and cast aside when he was done with her. Look at how he treats her its very much an abuser/abused relationship and with a psychopathic mind like his others don't really matter except in what they can do for him.
Technically, she was his wife. I suspect, though, that the word "technically" has been misused in the question. In terms of how he treated her, "a possession" probably describes it most accurately.
If they were married (as indeed they are appear to be), she can' t be a concubine, because the word refers to a woman who lives with a man to whom she is not married, as though they were married. On other hand, it's not a good idea to point this out to your live-in girlfriend....
She was his wife, just as every other abused wife is. It's not just a technicality; the law works that way for a reason. If you get free of an abusive spouse, even if you get the monster sent to jail for what he did, you're still married until you make the choice to get a divorce and follow through with it. Besides that process being a good thing (as a way of taking control of your own life, etc.), it also means that you have the legal rights of a spouse, and everything that comes with it (including access to bank accounts, etc.) while you're getting things sorted out.
Of course she was also his slave for the months that all of humanity were his slaves.