As far as I can discern, Launders and Venturer were not specifically dispatched to sink U-864. Venturer merely happened to be the boat on patrol off Bergen at the time that U-864 was returning to the port due to engine trouble.

Venturer was, at the time of the sinking, on her 11th war patrol. Her tenth patrol had been cut short, due to an engine failure after an attack on a convoy off the Norwegian Coast. The tenth patrol had only lasted six days; her typical patrol length was closer to two weeks. A full patrol would be followed by about a month's worth of rest, repairs, refits and exercises. In this case, though, Venturer was free to set out shortly after repairs were made to the troublesome engine. Had she made her full patrol, she would not have been present off Bergen when U-864 passed through, with another boat being in her patrol zone. Bergen, as a major port and U-boat base, had been a target of interest for the British submarine service ever since the Germans had occupied it. There were near-constant sub patrols in the approaches to the port, and it merely happened to be Venturer's turn to take these in February 1945.

Venturer set out from Lerwick on the 2nd February 1945, and arrived off Bergen on the 4th. Shortly after arriving, she received a message from headquarters in Dundee. This stated that U-boats typically used a northerly route to travel in and out of the base, but did not indicate that any sailings (or returning U-boats) were likely or expected. This may, however, have been a security measure, to ensure that the Germans were not tipped off to the sources of British information. She headed north, and began to patrol off the island of Fedje, in the vicinity of the U-boat transit route she had just been informed about. U-864, meanwhile, left Bergen on the 7th February, carried out a short exercise in the inshore waters, and began her patrol proper on the 8th, heading northwest. She passed through Venturer's patrol zone without being detected. Shortly after this, though, one of her engines began to break down and she was forced to return to Bergen. This once again took her past Venturer, and this time she was detected, attacked and sunk.

The sub patrols off Bergen were merely one of a series of barriers that any U-boat wishing to enter the North Atlantic had to face, whether heading for Japan like U-864 or to attack convoys. The North Sea, away from the Norwegian coast, was dominated by Allied air power; aircraft equipped with radar and Leigh Lights could easily detect and attack any U-boat operating on the surface. Those equipped with the more modern centimetric radar might even be able to find boats that were snorkelling. There were surface patrols in the narrows of the North Sea exits, and a major mine barrage blocking many of the northern routes out of it. Hunting groups built around escort carriers and directed by communications intelligence from direction-finding and Ultra intercepts could rapidly find and attack U-boats. Had Launders and Venturer failed to sink U-864, the odds were heavily against her reaching her destination.