Yeah I think it's safe to say we'll see a launch alongside the US. Hopefully there'll be enough for everyone.Check this article; https://www.macrumors.com/2015/03/01/apple-watch-selfridges-shop/
I'd say its a safe bet itll launch in the uk from day one, along with a majority of the countries that gets iphone at the same time. Considering the fact that the uk is a major market, it would be daft not to.