Have UKPC actually refused to cancel the ticket? If not, I think you're getting all worked up about this for no good reason. They almost certainly will cancel it if you send them an email with a copy of your permit and "Sorry, it wasn't displayed because it fell off the dash". If they don't, ignore everything and just send a copy of the permit and your tenancy agreement as your defence if they want to take it to court. There's no loss here, so they'll never win.
I'm surprised RMG are communicating with you because you're not their customer, your landlord is and they should be directing you to your landlord.
The land will be owned by a freehold company who contract the management to RMG. Whether RMG have the right to enter into contracts with parking companies depends what's written in the agreements between them and the freeholder.
It's no surprise that RMG don't have the right to cancel to tickets in their contract - precisely because they don't want to be wasting their time dealing with petty disputes over parking like this!