Anyone who exports animal products will from now on also have to have them checked by veterinarians. Permits for transport and cargo also differ in the UK, as do those for VAT returns. Products will also need new certificates to be able to be exported (proof of origin and development) as the UK is going to introduce its own certificates that will not be valid in the EU and vice versa. The same applies to those who import from the UK. Companies employing UK staff may also need to apply for a new work permit.
It all adds up to a lot of paperwork that will slow down trade between the UK and the European Union and add extra costs.
In addition to all that extra paperwork, there are notable rules that make trading more difficult and costly. For example, European companies that export to the United Kingdom will have to use different pallets than they are used to in the EU. That too is an extra investment for companies.