The issue is the quantity of paperwork, (as someone who works in the industry). We have to file up to 700 forms per container, and the authorities are pedantic, some must be on green paper, some pink etc. get it wrong, the whole container is rejected. With perishable goods that gets expensive quickly.
The system has operated for goods to NI for the last year, and been almost impossible to operate on any scale, hence food shortages and ultimately reduced ranges there.
It is a broken and unworkable system designed by government official who have no idea of the real world.