https://www.bbc.co.uk/news/uk-46564884
Following the UK's withdrawal from the EU, British citizens will be required to pay a €7 fee every three years to travel within the EU via the European Travel Information and Authorization System (ETIAS). Apparently the system is set to go online in 2021.
This doesn't affect us imminently, nor is it a big fee given the length of time it's valid for - but it's worth knowing. I guess it'll work in a very similar way to the American system, if anyone has travelled there.