one form thereof

a serious question because we don't know what the future will hold and negotiations are now so passé for the media and the gov have found an issue to hide it behind.

Norway has close ties with the EC but has a system of carnet that uses on-line registration etc. But lorries still get stopped and searched.

Basically, musicians found this to be a problem even in the 80s, and with ever increasing volumes of sound equipment it can only be more significant. The carnet says the equipment you carry is for use and not for sale. Without it customs in other countries could and did to refuse entry.
I came across this taking spares to swap in, in the event of successful repair of our company's machines.

So without prediction of the actual UK/EC decisions on borders I give this as a warning.

Be aware carnets may be needed in 2019. Don't assume - investigate!

Remember - no self-respecting guitarist can survive on a diet of only one guitar!