I'm almost certain that commercial SMS messages in the UK are illegal under the 2019 pornography regulations as you have no way of verifying the receiver is over the age of 18 along with the 2001 (maybe?) communications act as it is likely to be considered 'offensive' and the obscene publications act (from the 50s or 60s).
You can likely find the full text for all on
https://www.legislation.gov.uk