"False flag" smuggling was contained under the Navicert system. Even if a ship flew the flag of a neutral country, and was travelling to a neutral European port, the British Navy inspected it for wartime materials.
https://en.wikipedia.org/wiki/Blockade_of_Germany_(1939–45)#The_Navicert
So as soon as a "false flag" ship didn't obey the Navicert, the British would know it was German. Or as soon as the British inspected the "false flag" ship, it wouldn't be too difficult to uncover that everyone on board was German
Practically IRL Germany was shut out of oceanic maritime trade during both world wars. There is a big difference between the raiders you mention and cargo ships. Cargo ships are much larger, much more noticeable, much slower, they need to refuel much more often. Trading with Sweden across Kattegat was fraught, especially in the late war, even with Norway and Denmark occupied. And German really wanted to trade with Japan, but using surface ships was simply an impossibility. So much so that Germany resorted to using cargo submarines which is an extremely inefficient way of transporting materials.
And the German surface raiders weren't particularly successful. Here is how the German fleet fared in the first months of the war:
It doesn't seem to me that the British had too much trouble with the German merchant fleet. The German merchant ships caught in the open seas when the war broke out didn't have anywhere to refuel and couldn't make it home.
These problems were largely due to the fact that the German U-Boats were hiding from the British Navy. The Kreigsmarine didn't have Aircraft Carriers to launch patrol aircraft, or really any surface ships at all to pinpoint these convoys. These are obstacles the Allies are unlikely to have when searching for hypothetical German convoys.
Ok maybe something like 1% chance of success, and even
that is pretty generous, with no chance of breaking even on the investment. If historically there were a reasonable chance of success for surface blockade running, the Germans wouldn't have felt the need to use cargo subs.