There are some conditions:
"Premium for up to 6 people residing at the same address for £14.99. One device per primary account holder, while supplies last. Sub-accounts ineligible. UK only. Shipping & handling by Google: a Google account and Google-approved form of payment required to redeem a...
Apologies if this has already been posted.
https://www.spotify.com/uk/family/
Just followed the link myself and have just received my confirmed order email receipt. (the £49 discount is applied at the checkout).
Works for existing subs too.