I'm not sure this is legal. From the UK government website
https://www.gov.uk/marketing-advertising-law/direct-marketing
'5. Direct marketing
You must check if customers want to be contacted by fax, phone, post or email, and give them the chance to object.
When you collect customer details, you must get their permission if you want to send them other offers or promotions.
You must also ask for their permission if you want to share their information with another organisation.'