I am not sure if Brexit is effective which would mean we can no longer get into the Louvre museum in Paris free with an EU passport, instead and having to pay the €15/17 euros. I want to visit today and yesterday they were at capacity. We could book it online but perhaps we will just go if we can get in without paying. I am in the 18-25 age range and a UK citizen.
The Louvre admissions page says
18-25 year-old residents of the European Economic Area (EU, Norway, Iceland, and Liechtenstein) enjoy free admission to the museum year round.
Britain has not left the EU, so this would still apply to you. The page does not indicate how yoou demonstrate your residency (I can have a UK passport, but am not a resident) nor how you claim the free ticket. It also doesn't suggest that this somehow gets you around being "at capacity."