To get it, you just need to enter the code B-GOOGLE21 a checkout (in the UK), but note that there are some restrictions. Stadia pre-orders are excluded across Europe, and in France, the Pixel 3a isn't part of the deal. Subscriptions to Nest Aware are also excluded. Note that the promo code appears to be different for each country, as well, and you need to have an address in the purchasing country.
All told, it's a rare opportunity to get Google hardware deals apart from the usual sale times around Christmas and Thanksgiving. Also, if you're planning on buying a Pixel 3 of some kind, be aware that Google is launching the Pixel 4 sometime in the fall. We know exactly what to expect from that phone, so if you need the improved camera and other features, plan accordingly.