BS EN14764 states:
The brake lever for the front brake must be on the right- hand side of the handlebars and the rear brake on the left-hand side.
That’s the construction regs for bicycles in the UK, so yeah, I’d say illegal. It’s maybe an overly strong term, but it covers it
Which is a very basic thing that any Cytech qualified mechanic will have been taught.
They sound like a bunch of chancers to me, they certainly aren’t taking any pride in their work.
Plenty of companies source bikes from Europe when UK stock us low. You change the brakes round. That’s just what should happen. I’ve done plenty, road bikes, hydraulic discs, whatever.