It is detective work.
FreeBMD has the birth, marriage and death indexes, and is free to search ( a lot of the genealogy sites also have them but are not free to search.)
This is their search page - www.freebmd.org.uk/cgi/search.pl Put in what you do know about her, and guess at the earliest and latest that her birth was likely to be. If you know where she lived when she was very young, you could also select the place, as she is likely to have been born there. If not, leave it blank, or it will restrict which records are searched.
You will get a list of possible results, with the year and the quarter of the year when they were registered. This could be several weeks after the actual birth.
If there are a lot of results for that name in the years you searched, you may be able to narrow it down somehow - perhaps by searching in the marriages for her and comparing places with the births, and/or searching in the deaths for her under her married name and noting the age of each result you get, then comparing that with the year of birth of the results in the births search..
When you find a likely result, take a note of the reference after the entry and use that to order a birth certificate from the General Record Office. It will cost you less if you have the reference than if the staff there have to search for it.
The route is not instant, but when you have found it you will have a thrill of triumph - and you will see why genealogy is so addictive.