Get them while you can!

From October 22nd through November 18th you’ll be able to get a code for a Shiny Zacian in Pokémon Shield and Shiny Zamazenta in Pokémon Sword at GAME stores in the UK. This follows the announcement of the shiny Zacian and Zamazenta distribution in the US through Gamestop.

So far GAME in the UK is the only European distributor to be announced.

Source: Serebii