1- Rules of Origin
British firms must show where their goods were made and where each component in those products came from to avoid tariffs being levied on goods into the EU.
For small companies trading with the EU this is a nightmare. If you are trading electronic goods you'll have dozens of components from different countries integrated into one finished product. if you have to account, record for every product, it's unviable.
British exporters must register to pay VAT in EU nations immediately, each target country individually.
Again for small companies, it's not worth the paperwork, cost of time.
We're having to apply for a UK VAT ID to sell into the UK, it's not difficult to do. Before Brexit there was a sales threashold that if you sold into the UK a value of goods that exceeded it, you needed to be registered for UK VAT anyway (same in the other direction). So again, it stuffs the little guy and sales to countries that don't meet that threashold. I think I read that level was going to be reduced throughout Europe later this year to 15,000 EUR per year anyway, so most companies would have to obvide by it.
Most decent sized companies it's a hasle, but accept it will have to be done.
4- Dueling Certifications
Products sold in the EU require a CE mark showing they meet health, safety and environmental standards. Because the U.K. wants to develop its own certification, companies selling in the EU will need to register for both standards.
I wrote on this forum many times about CE marks, EMCC etc. It's exactly as I said it would be.
Right now it looks like the UK will effectively copy the CE requirements, so it'll be the same tests that are required, but double the amount of paperwork will be required an the additional UK mark will need to be attached to products.
A pointless extra level of legilisation, where the UK helped setup CE marks in the first place.