We did drill with the police cadets on a few occasions (mostly teaching it to them), it is just like any other activity. Each “side” would need to send its own “CFAVs” to look after their cadets and whoever is delivering the activity should be qualified to do so.
MOD indemnity would apply for CFAVs delivering an authorised activity for non-members, as they are a third party and this is covered under JSP 814 - this would effectively be PLI. What it would not do is cover you for activities with a non-MOD cadet force except for claims against MOD CF members. The non-MOD CF would need to have appropriate insurance for their activity for this. Should you require compensation for injuries sustained on an activity run by the fire cadets, your claim would be against their insurance, not the MOD.