I am a professional genealogist, and, agree with many of the comments here - the 1939 Register is worth using, but, I find it's greatest use is to establish a date of birth - providing the person being recorded gives the correct date of birth at the time!
My Grandfather was younger that Grannie - but pretended he was the same age - to the extent that even though she died in 1926, he let the family celebrate his 90th birthday in 1969 - when it wasn't!
I couldn't find him on the 1939 Register, even though I knew where he was, and we have a very unusual surname - at this time he was married to his 2nd wife who was 15 years younger than he was - guess what? he had done it again - now instead of pretending he was older to match his wife, he was now pretending to be 8 years younger to match his 2nd wife!
The point being - no genealogy site is 100% effective for many reasons - I subscribe to many of them - because there are always our ancestors out there putting a spanner in the works anyway!!
Have fun!! and don't forget that google is the best tool. Think outside the box.