You dont need a licence, by law to operate any powered access.. its like PPE, all sites/companies have different rules. Some require you to only wear hi viz vest, others require hat/gloves/goggles/vest/blah blah.
Powered access is the same, some sites dont need proof, others need a ipaf ticket before you can use any equipment. IPAF tickets arent a licence, they are a certificate of competency. IPAF is a training and competence company, they arent a licencing corporation. Told to me by the guy who trained me at IPAF.
Insurance, on the other hand, is another story alltogether. You can phone your PL insurance company and ask them to cover you for specific times, but im assuming your ins comp will want proof of training. Some hire companies (Not HSS) provide insurance but its very limited so ask as many questions as possible..
Or just chance it..Why not just get your IPAF licence? its a one day course, then your set for 5 years.. as mike said, spread the costs, then when you have your licence, you can go in for more high level work and make it pay..