Thread by @vivamjm

The concept of a customs union & single market I always thought were easy: it seems not. Again just been in a conversation where not..& explained it (successfully) like this..
Imagine a shipment of o/seas expensive watches, or used cars, or a ton of beef enters a port in Wales

The shipment clears customs after satisfying Welsh border force it's met the relevant (rules of origin) tariff/duty & VAT rates. Once cleared (or originating in Wales) does (did) the same shipment need the same costly process at border with England or Scotland (or NI)? No!.. Why?

Because they are part of the same customs territory (a customs union) .
For nearly 50 years this extended ever further into continental Europe and Ireland...but retracted back to just UK (nearly all UK; NI is a bit complicated).. this year

Similarly
..shipment is released onto the market after satisfying Welsh border force it's met relevant UK (bio) standards & compliances. Once cleared (or originating in Wales) does (did) same shipment need same costly process at border with England or Scotland (or NI)? No!..Why?

Because they're part of same regulated/policed standards territory (a single/internal market). For nearly 30 yrs this extended ever further into continental Europe & Ireland..AND further still into the EFTA4 countries ..but retracted back to just UK (for goods GB only)..this year

Similarly
An individual clears immigration passport/ID control after satisfying Welsh border force he/she has met relevant UK entry conditions. Once cleared (or originating in Wales) does same individual need same process at the border with England or Scotland or NI? No!.. Why?

Because they're part of same common travel area territory..
& has long included Ireland (a "Schengen" zone).
However this has *never* extended ever further into continental Europe ..
UK & IRL opted out.
(Although Single/internal market means the individual has FoM to work etc)