Carnets
A Carnet is a “passport for goods”, (NOT MERCHANDISE FOR SALE )
If you are moving professional equipment (such as musical instruments, backline, video/photographic equipment, exhibition goods) between UK and EU on a temporary basis (for a concert tour, for example), you will need to declare them to Customs using an ATA Carnet. This document acts as a passport for the goods, enabling you to import and re-export them without payment of duty or VAT.
The Carnet consists of a booklet of vouchers and counterfoils which is presented to Customs at each border, enabling you to declare your goods for duty-free and VAT-free entry and exit. After use, the Carnet must be returned for discharge by the Chamber of Commerce by which it was issued, whether in an EU country or the UK.
Please remember you can use your ATA carnet anywhere in the world where one is required.
-via ukeartswork
Information listed in alphabetical order

ATA Carnet
Application website for an ATA carnet (equipment) from the London Chamber of Commerce and Industry

Gov.UK
Information on different types of licences (including carnets for equipment and vehicles)

Musicians' Union
16 February update – EU Touring Update: Musicians with Portable Musical Instruments Exempt from ATA Carnet
It has been confirmed today by DCMS Minister Caroline Dinenage MP that musicians travelling into and around the EU with portable musical instruments are not required to obtain a Carnet.
11 February update – MU Secures ATA Carnet Discounts for Touring Musicians
Whilst the MU lobbied hard against Brexit, the agreement reached with the EU means that currently ATA Carnets (a travel document needed for instruments and equipment) are a reality for musicians working in the EU.
11 February update –Musical Instrument Certificate (MIC)
What the Musical Instrument Certificate is for, in which cases is it needed and how musicians can apply for it before travelling.
National ATA Carnet Unit

UK Europe Arts Work
Basic Q&A about carnets. What are they? Do I need one? etc.