But what about Brexit-related trumpet issues, like carnets and the like?
I have no idea what a carnet is!
A carnet de passage is a customs document that allows you to bring valuable items into a foreign country without paying customs duties - if you declare that these items are for personal use only, will not be sold and will be removed from the country again when you are leaving. If there were no carnets, any instrumentalist travelling to a concert would have to pay full import duties on any instrument they are going to use there. Imagine a violin player with a multi-million-dollar Stradivarius...
Within the EU, there is free travel of people and goods, so you don't need a carnet. But whenever you leave the EU, you have to fill in a carnet de passage on leaving your home country exactly describing your instrument (maker, brand name, serial number - some rotary trumpet owners regularly get into difficulties because the smaller makers don't bother with serial numbers - case, number of mouthpieces etc.), present that at the point of entry into your destination country where every detail will be checked, answer questions about your purpose of travel... and if you are getting paid for a concert (or even just intend to hand a hat round) you will have to apply for a working visa... All that kind of thing will be unavoidable on the borders between the EU and the UK... unless some kind of working agreement can be found.