Since the United Kingdom left the European Union there are a few things you should be aware of when towing to Europe with a trailer.
A carnet is a temporary export document covering, amongst other things, vehicles and tools/spares you might want to take with you for an event. It’s a long established system that until Brexit, western European trackday enthusiasts didn’t need to be concerned about (except Switzerland!)
In summary;
If your car is road legal and registered to you, then you do not need a carnet to cover that vehicle.
However – any spares/tools (spare wheels, jacks, spare parts etc) with you? These DO need a carnet.
If your track vehicle isn’t road legal then it will also need to be included on the carnet.
ATA Carnets in the UK are issued by London Chamber Commerce & Industry (LCCI)
This is the official website and text for ATA Carnet :
https://www.londonchamber.co.uk/international-trade/trade-documentation/ata-carnet/
And from the FAQ on this page;
The ATA Carnet is a temporary admission document which acts as a passport for goods for three main categories of goods:
(** Specially adapted vehicles can be driven on public roads and include any vehicle with permanently fitted equipment (i.e. broadcasting vans, exhibition trailers, water filter trucks etc) )