Don't need to catch them all, just at a steady enough rate to be a real deterrent. And preferably deep in what will be UK waters, so they can't plead minor navigational error.
After a number of trawlers get impounded, the number trying it on will drop off.
It would also be prudent for UK trawlers to stay well away from EU waters - Marcon in particular will be itching to impound them in retaliation.
I hope you're correct. Fishery protection patrols in the early 90's, catch a Spanish or French trawler bang to rights. Double netting or hidden over quota fish. Arrest and escort to somewhere like Dover for a court appearance. Result! Big 20 or 30 thousand pound fine and/or nets confiscated. Then find out the fines are all paid from their home port's central fund. Obviously the profits outweighed the risks. Hopefully these slush funds will be diminished.