I don't wish to throw a cat amongst the pigeons, but the d/o/b on the 1939 register doesn't always match the d/o/b on a birth certificate.
I've encountered multiple occasions where I have the birth cert. and once I 100% accurately locate that person on the 1939 register, the d/o/b doesn't match. Usually it's close, for example, the 3rd May instead of the 3rd of July.