My husband's entire extended family are known by their second names, it's a common practice in many areas,
As most of them have the same first names, and same surnames, the tradition is that they go by their second names, or you would have a lot confusion.
Just use the first initial, second name and surname.
so David Henry Smith, becomes D.Henry Smith,
Then no legal change is necessary, my husbands bank accounts , have the equivalent to Mr.D.Henry Smith, everyone knows him as " Henry", schools always ask what a child's chosen name is, when they start.
The point of having second, third names is so you can use them.
Agree with the above. You don't have to do anything other than to start using it! People will catch on soon enough, and if you want to do it, do it now. The longer you dither, the harder it will be, and then it will be too late (speaking from experience). I would say go for it, or you'll regret it for years to come. I know quite a few people who go by their middle name. It's not a big deal, really.
My dad was always known by his middle name as his unimaginative dad had named him after himself, what ego! As in the same first and middle name, so granddad was known by my dad's first name and my dad by granddad's second name.
You can change it at the register office up until one year and the new name will appear on birth certificates - if I remember correctly (personal experience), you only pay for the new certificates. After the age of one it has to be done by deed poll.