First - full age will mean over 21
So, if Margaret was 20 in 1843 she was born around 1823. That is before Civil Registration began so there will not be a birth cert, just a christening record in a Parish reigister, if she was christened.
But before you can have any hope of finding that, you need to know what sort of area she was born in.
have you found her in a census after her marriage? If so where/when does she say she was born? If not, that would be the first step.
Have you found her on 1841 at all?
It will be impossible to pinpoint any marrige for her parents until there is an idea of where and when - she may have been born 9 months after the marriage (or dare I say less) or 20 years, dpeending on if she was the eldest or youngest.