The free google home incentive is currently only available in the US for Family subscriptions of $14.99, i believe this should also be available in Europe / UK as we pay a higher cost for the same service (in UK spotify family works out to $19.40) with none of the benefits such as this.
Updated on 2019-03-21
Thanks for voting on this idea.
The Google Home Mini offer is now currently available in the UK and France. If the offer becomes available in other countries, we'll let people know on Spotify.com.