Ubisoft+ it seems to be available now on Google Stadia in Europe, after being launched as a beta on this cloud gaming service only in the US, therefore the specific subscription for Ubisoft titles should be accessible and usable by everyone even in these parts.
To tell the truth, the official Google Stadia website still reports the communication that Ubisoft + is available in beta only in the USA, but according to various reports, such as that of DailyGoogleStadiaNews, the service is also active in Europe, but you must log in through the site Ubisoft.
Once you have logged into your Ubisoft + account through the official Ubisoft website, you should be able to connect by going to the Ubisoft subscription page on Google Stadia. This unlocks thefree access to games of the French publisher within the Google Stadia catalog, based on the provisions of the subscription.
As for Ubisoft +, it appears that the company is thinking about different price ranges with different tiers, based on a survey, while we are still awaiting any news on the famous Ubisoft + issue on Xbox Game Pass, with Microsoft trying to close the agreement but official news on this front has been scarce for a few months now, except for news arriving at E3 2021.