It's a good tip!
The dates we are looking at in November are £137 cheaper on the French site because of the children under 12 free offer, and that's with the UK site having a 25% off.
Has anyone in the UK booked via the French website, or is it only over the phone?
you can book online using any country's version of the website you like, but if you book online, you pay in full when you book. there is no problem with the confirmation etc, it comes through by email, and the confirmation number is all you need to check in at your hotel.
It depends which offer is out on each country's website, whether the UK, France, or any other country has the best deal for your dates when you look. Its not always cheaper on the French version.