Doing travel for a motor-racing team pre-EU, they needed a carnet to attend Le Mans. Fill it out, show it to customs on the way in to France and again on the way out and so long as you take out everything you took in there was no issue. If they dumped their spent tyres at the track's tyre dump they would be charged import duty for those tyres. They only did that once...
This is the worst case scenario of what could happen after the transition period, a carnet to enter to the EU, which would have to be shown at point of entry and exit from the EU, not as the hysterical petition suggests crossing from one EU country to another. Will add around £150 to the cost of a tour for a band the size of editor's.
Chances are there will be some kind of arrangement that means we won't return to that system though. But no one knows and won't know until the end of the year, at the earliest.