Taking my Prius from UK to France : headlamps

(Sorry about the treble post - I got error messages after my first two attempts saying that the server was busy, try again!)

This is a post about headlamp deflectors.

It's my first post here so I am not sure which forum to put it in. I hope there are plenty of UK-based Prius owners on here.

I am driving from the UK to France on holiday in a couple of weeks, for the first time. So it is a French legal requirement that I install something to deflect my headlamps from their UK driving-on-the-left-of-the-road settings.

So I asked my Toyota garage. They can sell me a headlamp deflector which presumably goes inside the headlamp, but they advised me that in addition to fitting that, they also need to actually deflect the headlamp beams, and it is best to get the garage to do that. But what about when I come back, I said. Do I have to book the car in again to get this reversed? They said no because it is legal to drive in the UK with headlamps set up for Europe, so they suggested I leave them set up for Europe permanently.

This doesn't sound right - won't it be inconsiderate for other road users?

Does anybody here have any experience of this? Do I need to do what my garage suggests or I can just buy something that sticks on to the outside of the headlamps - which is sufficient for most makes of car.