What 30 Commando did during the war is to advance ahead of conventional units and gather intelligence, particularly maps, codes, ciphers, enemy intelligence documents and dossiers, the sorts of things that conventional troops might ignore or, worse, simply destroy. In this respect they are a lot like modern small unit special operations teams, such as the teams sent into Iraq during the first and second wars to pinpoint targets, observe and report troop movements, and so forth. Intelligence operations, rather than combat operations.
This is 30 Commando's connection to writer Ian Fleming; as a Naval Intelligence Officer, he would pick targets likely to possess valuable intelligence assets, and have elements of 30 Commando attached to conventional units that would then strike (or be striking regardless) those targets, allowing 30 Commando (at this point known as "30 Assault Unit") to snatch up all the valuable intelligence and send it back for analysis. The movie trailer seems to make it out as if Fleming is the creator or leader of the unit, which is wrong; he was merely the man (or rather, one of the men) who picked their targets and planned out their operations.
One of the more pondered and notorious activities attributed to units like 30 Commando (it is not unique in its operations by 1944, with the creation of the even more notorious "T-Force") is the seizing of German technology secrets; many conspiracy theorists and speculators think units like 30 Commando and the Target Force procured top-secret Nazi "Super-Weapons" and other advanced technologies before the Russians could get to them, or before the Germans could hide or destroy them. Apparently, Fleming wrote his character James Bond into such a unit during World War 2; a suitable unit for a spy who would go on to deal with all manner of technologically advanced villains over the course of his career as an agent.
In any event, here is the trailer for Age of Heroes (2011):