From: Jack Campin
Date: 26 Dec 20 - 05:43
. . .
It's as bad as the worst scenario anybody envisioned.
. . .

Not really. You only have to look back a few days to the screed posted by "C21st Primative" to see that things have already moved on. But more will surely come to light.
If you play an instrument, you’ll need an ‘ATA Carnet’ for it, to cross the EU border and any within the EU thereafter. This applies to all kinds of professional equipment you may need
Why will you need a carnet if neither side will impose tariffs on goods being traded, and a zero quota agreement would mean no limit on the quantity of any type of goods that could be traded.

You get an ATA Carnet from the London Chamber of Commerce & Industry and they cost £351.60 each (or £562.80 for their express 2 hr service) and they last 12 months

This is for each instrument/item. Pity the sax player who travels with sop, alto & tenor in her bag, that’s just short of a grand a time. Imagine the bill for an orchestra. Let’s hope those gigs are well paid!
The sax player would only need multiple carnets (if they were even needed) if they were following the suggestions here. A carnet can cover multiple different items for a single fee: Gov.UK

This page at Gov.UK even seems to suggest that paying for a carnet is not necessary if you're willing to do a bit more of the paperwork yourself:
Get an ATA Carnet
You can use an ATA Carnet in around 70 countries.

Countries have their own rules about what goods you can bring in with an ATA Carnet. Check with the issuer in the country you’re exporting to.

If you cannot use an ATA Carnet (or do not want to pay the fee), use a Duplicate List to temporarily export your goods instead.

If for any reason the equipment/instrument you take with you is going to stay in the EU when you come home (ie you sell/rent it/give it away), you will need a C88(SAD) export declaration (just 8 parts, 12 pages to complete) from the UK Govt
Again, if there are zero tariffs, and no quotas, Why?

I would suggest that people wait until this has passed through Parliament, and all the ramifications have been looked at, before trying to spread panic based on individuals assessments which were made before a deal was agreed.