This is not legal. An unsolicited text message, even if it is asking whether you want to view an ad, is still an unsolicited text message.
Actually, reading
here shows that the technique is using Bluejacking. I don't think this differs in any way from providing an unsolicited message -- it just happens to be via a fake Bluetooth contact.
Lousy spammers always say things like "you can always opt out". The point is that all current legislation requires you to opt-in actively, not by being sent marketing messages saying "would you like us to continue spamming you"!
In any case, having Bluetooth turned on all the time is rather daft if you don't need it: it drains power and is a security risk. Still, I may punt this one to the Information Commissioner and see what they have to say about it.
BC