I’ve done a bit of digging.
The domain in the SMS was registered for a single year back in February – the hallmark of a site that knows it’s not going to be around for long (ie, probably a scam).
The site says the service is provided by “PM Connect.” pmconnect.co.uk has been around since 2014, which would imply that what they’re doing whilst morally bankrupt probably isn’t technically illegal.
Googling PM Connect tells you all you need to know very quickly, the Internet is awash with complaints. They’ve been pulling this trick for years, click-through adverts which sign you up to subscriptions just cheaply enough that you might not notice them on your bill.
If you don’t reply, you will almost certainly be charged. Ie, the SMS isn’t the hook, you’ve already been hooked.
They list a contact email as wwe@mpay.co.uk – this domain was registered in 2012 (by PM connect).
The phone number on the SMS is also listed on the web page as their “helpdesk.” Calling it – and indeed, any 03 number – isn’t premium rate but will be billed as a regular geographic number, so is safe to call.
If it were me I’d call them to give them a bollocking, but it might be easiest just to do the STOP thing (try STOP ALL rather than the wwe it suggests).
If you’ve already been charged, the Phone-paid Services Authority is the regulator here. https://psauthority.org.uk/for-consumers/ive-been-charged-what-next . They also have a service checker which you can enter the short code into, this confirms the above information so the phone number is likely legit (though it lists a contact email of help@pmconnect.co.uk instead of mpay.) You can also report them from there.
HTH.