No, you can't conclude she didn't. If she was young, there's a good chance she was in the NHS system from birth. So check the GRO index, that could tell you if she married, but the marriage may not be where expected. Look through the emigration records. If she's not there, she may have married in Scotland. Look at the probate calendar for a will for her parents and siblings, to see if she's an executrix (I've found a few marriages that way). Finally look for a death for her from the time she was included in the National Register onwards.
After all that, she may have lived with her man as though married but without having a little piece of paper saying so.
If she was young enough to come under the NHS system from birth she will not appear on the 1939 Register the NHS did not start until 1948 and the available register was taken in 1939.
Anyone born after that would be on the later register which is not available.
If she was living in England & Wales at the time of the 1939 Register and married in Scotland at a later date the England & Wales register should have been updated with her new married name.
My mother who was married and living in England at the time of the 1939 Register is shown under her 1st married name (married 1938) which has been amended with the addition of her second married name. Her second marriage was in Scotland on 21 Oct 1946 and the date of the amendment 25 Nov 1946 (25/11/46).
My brother's entry (by my mum's first marriage) shows his name change in 7 December 1957. I can only assume this was when he registered at a new doctor's surgery in Edinburgh when he went up to university as he did not formally change his name, but changed it by use after her second marriage.
Do not forget systems were far more relaxed in those days, identity was more about recognising people (patients) by sight and many people only visited a doctor occasionally. Many were used to having to pay for medical treatment and so did not visit a doctor for years at a time.
If a doctor knew the person in the community rather than as their doctor he/she may not think to amend ther medical notes, I believe this to be the case with my brother as we lived in a village where everyone knew everyone else by sight.
Cheers
Guy
PS My brother's record is open as though he was born in England in time for the 1939 Register (13 Sep 1939) he drowned in Scotland in 1958.