If the California stores ships to the UK, you can be certain that they will.
And they'd be right to do so as a country has sovereignty over what is allowed or not in their country, not matter the country of origin of the seller.
They have a right to stop the packages at the border. They have no right to go after the stores themselves and more importantly they have no power to do so unless the US helps them, which they would never do over a free speech issue.
And they'd be right to do so as a country has sovereignty over what is allowed or not in their country, not matter the country of origin of the seller.