OFCOM dont regulate premium rate services, the government agency PhonePayPlus does (Sounds like a dodgy company but it is indeed the correct angecy) - their website can be found here:-
Welcome to PhonepayPlus the regulator of phonepaid services
You should raise the illicit text with your service provider first as with all premium rate services in the UK the actual majority of funds paid to the vendor (the one sending the messages) is witheld in full for 30 days and realeased in portions of 90 days.
SPAM messages like this are illegal unless you have somehow requested this service, there are tight regulations about hiding opt in and costs for these services in small print etc so you must explicitly have been made aware of what you are going to be charged for beforehand - otherwise it is illegal and should be refunded - chances are your not the first person and your service provider already has established a protocol for dealing with complaints from this source.
Good luck and let me know if you need to know more detail (the company I am consulting with operates is one of the major premium rate service providers).