Okay this may get complicated, the CTA itself is not legally binding, but the laws that implement it are. Basically the CTA predates the notion of legally binding international agreements, and so is just a piece of paper signed by a few countries a century ago, but those countries put provisions for it into their own laws. So what Wikipedia says is literally correct, but not correct in practice.
Also, the British Isles and the UK aren't the same thing, The UK is a country made up of the four countries England, Scotland, Wales and Northern Ireland. The British Isles are an island chain in the North Sea made up of Great Britain (the island where England, Scotland and Wales reside) Ireland (the island where the Republic of Ireland and Northern island reside) and various smaller islands like the Isle of Man, Jersey, Guernsey, the Isle of Wight, etc.