But you most likely have a passport, and most (western/european) countries have "automatic" visas to the UK, so for the most part, training from London to Edinburgh isn't much easier than Paris to London. Sure there is no passport control, but UK passport control is typically just scan your passport's chip and look at a camera.

I work with people who actively chose to go to conference in Berlin instead of the UK due to visa issues. The passport control is a real issue one cannot just handwave away.