I'm a freelance and an MU member, hence I have the union PLI cover. However, this only applies to me as an individual and not to a whole band in which I am performing. A couple of bands I do function gigs with now have a policy for the band. I'll try to find out where they go for these.
Another reason for MU membership is access to insurers who will give you a decent quote for vehicle insurance (if such a thing exists!) as part of a professional body.
On PAT testing, as far as I understand it, any Portable Appliance (i.e. amps etc for us, kettles and computers etc in a standard business) in a place of work is supposed to be checked. Last time I had gear tested, the tester said the hardwired Fender amp was to be done, but for my Trace head only the kettle lead needed doing as that's the bit that actually plugs into the mains. It appears to me that, rather like Building Regs, the rules are open to interpretation. Any surprise we get wound up about this issue? When it comes down to it, I'll still frequently check inside my own plugs looking for any loose or broken stuff, as 12 months is a long time in the life of a mains cable. A loose screw can mean an involuntary earth lift!