They'll be making some money from you and whichever two friends you find then Tigger. These modelling agencies make you up then take several dozen photos which they compile into a portfolio. That generally costs you several hundred pounds to buy. Multiply that by three of course with your friends.
Of course, if you cancel they have only made £30 for doing absolutely nothing!
About 4 years ago my wife and I paid for my wayward step-daughter to have this done. It cost us £500 for a portfolio of 78 photos. They were very good I must say, despite the fact she then just waltzed off and did nothing with them. It just seems this agency has approached you rather than you them. A good sales technique!