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.
Since the UK is not allowed to own them anymore.
Basically, the UK is required by treaty with RoI to not impose a border between the RoI (not part of the UK) and Northern Ireland (part of the UK). And the UK wants to erect a border between the UK and the EU. So, do they break the treaty with RoI potentially causing The Troubles [1] to come back? Or does Northern Ireland effectively leave the UK? Or does the UK allow the EU to require that Northern Ireland and the UK by extension to conform to the trade rules the EU made and will make?
They can only pick one, and they don't want to pick any of them.
[0] https://www.youtube.com/watch?v=J1Yv24cM2os [1] https://en.wikipedia.org/wiki/The_Troubles
If course the EU won't allow a backdoor into the common market, so they will be the ones pressuring Ireland for a hard border. Whatever hard border would be imposed solely by the Irish republic, which means the British government abides by the good Friday agreement.
Imagine Texas leaving the US, all the assumptions it invalidates, on all different levels.
It took 7 years to reach CITA deal(EU - Canada). It might take a decade before the UK catch up with trade deals. Even then it would not be as good as EU deals. Companies are already cutting investments and withdrawing from the UK as we speak.
https://en.wikipedia.org/wiki/European_Union_free_trade_agre...
This will obviously have some problematic impact short term