I just received an email from google play wih above subject "Festive apps and games + £1 to spend on Google Play" and there is a link in the email that says "Get a £1 credit, on us" and then there is a button to redeem it. Its in email so I believe it would be personalised code for everyone.
Check your email and enjoy the credit.
Hope it helps someone.
It's the season of giving so enjoy £1 towards your next purchase on Google Play. Click or tap the button below by 31 December to add the credit to your account.*