Well I'm sorry, but on a question board where there are several legal experts, not one of them has bothered to answer your question properly.
I was given my natural fathers surname on my birth cert. my mother then remarried when i was 3 years old and I took my step fathers surname, I was then known by that name for the next 40 years, my passport, driving licence, bank accounts etc etc, were all in the surname I took when my mother remarried. At no time was it changed by deed poll. I was married twice, and all I had to do was state that my name was ........formally .........
When my 3rd partner and I decided to get married in the Dominican Republic, because of the different name on my birth certificate, I had to get my name legally changed by deed poll to the name I had been using since being a child.
It took 1/2 hour at a solicitors and costs about £75.
Ignore all the comments about being suspicious, they are ridiculous, and posted in ignorance of the law.