The UK had trade deals with various countries (including those within the EU) as part of being a member of the EU. It has also built up a lot of infrastructure on the facilitated access to the EU this gave. This covers things like: 1. just in time transport of parts for car manufacture (and other things like florests getting flowers from the EU overnight) that are tariff free; 2. just in time transport of food and medicine (including radioactive materials for x-rays); 3. access to the EU air space for flights going to/from the UK; 4. seamless trade between Northern Ireland and the Republic of Ireland (including roads, houses, and farms that span the border).

In addition to this, a large amount of regulation comes from the EU, along with 40-60 years worth of continued integration and interdependence.

We are now ending that overnight. This needs adequate planning and prepration to ensure that things don't fall over. Things like border checks on both sides and agreements to make sure that things such as access to EU air space continue to run smoothly.

It is problematic because of the scale of the task and the lack of information/transparency from the government. So you have things like banks and aerospace companies moving offices to the EU to try and retain access to the EU on which they depend.

There are things like having to store information on EU citizens within the EU. This was fine when the UK was a part of the EU, but with Brexit that changes, so EU citizen data needs to be stored on servers in an EU country like the Republic of Ireland.