UK Executive Order? Nah... They will have a 2nd vote and rig it.
The Brexit vote was never legally binding. It will take many acts of Parliament to unravel the gawdawful mess that is the EU. Cannot happen overnight.
There all kind of implications. Contracts have been made on the assumption of one regulatory regime, and now they have another one, which no one knows or understands.
In immigration, visitors with EU passports are waved through, those from the U.S. and other non-EU countries get in line.
This is gonna be a mess, but better a horrible ending than horrors without end.