I'm not sure about Europe/UK, but in the US, the left hand controls the front brake as well as the front derailleur. The right hand controls the rear brake and the rear derailleur.
Personally, I like the front brake on the right hand. It's more consistent when I'm riding my bicycle or my motorcycle. (Right hand front brake)
On my mechanical disc brakes, it's easy enough to swap the cable over, leaving the levers in place. On my mountain bike with hydraulic disc brakes, it's not as easy. I could swap the levers, but then the bleed hole is upside down. To do it correctly, I would need to disconnect the lines into the lever and swap the lines. This would require a minimum a complete bleed of the system. It may even require new hoses, depending on your brake system.
I'm sure your mechanic understands this and is guessing that if you get use to it, you'll be fine because most bikes are setup this way. If you jump onto a friend's bike, more than likely it will be setup left front/right rear.
People hate borrowing my bikes because of my brake setup (Which is fine by me!!)