That might be true from the perspective of the allies and the way they fought wars. But from the german perspective, a direct assault on UK was far more effective than waging years of extremely costly and useless attritional UBoat & commerce raiding warfare. Further more a direct assault was much more in keeping with Blitzkrieg methods. Finally if the Germans were unprepared for an invasion, so too were the Brits.
The RN had only the ablity to patrol 1/6 of the seas around the UK so they erected blocades based on geographical chock points [English Channel & GIUK]. The UK coast was wide open to port to port invasion. They had just established a coastal defense force of trawlers ,backed up by about 90 older destroyers plus 'Dads Army' on shore...but these were just a shell. Only about 1/4 of the trawlers had large enough guns and in any event only 1/3 of these warships and auxiliary warships could be available at any one time due to the defensive nature of their patrols. By contrast the German fleet could surge their entire surface fleet [as they did in Norway]. Worse still only about 2/3 of the major ports had coastal batteries , while the raw recruits of "Dads Army" had ~ 1 days supply of small arms ammo to back these up. The Admiralty and Churchill knew this all to well , they assumed the Germans would first have to amass a hugh fleet of trawlers and barges prior to any invasion, which ofcourse would take weeks, thus giving away the element of surprise and time for Home fleet to respond. But the Germans were aware of this too and made preperations
In point of fact the germans planned for lossing upto 1/2 their invasion force before calling it a day. Had the operation been planned with the usual desception, it might very well have worked, especially they had not made the mistake of "Dunkirk" and thrown away the element of surprise with the "BoB" and their surface fleet in the "Invasion of Norway". There is no doubt it would have been a horrible fight anyway you look at it, with tens of thousands of casulties on each side per week, but I would put the Wehrmachts chances of over running the UK atleast 50-50...worth the risk.
The reason the so called bluff theory has only stuck around , is due to Hitlers odd irrational prewar belief that he could control the British reaction and avoid war with them altogether. Even the most elementry study of recent history convinced people like Admiral Raeder [German navy chief] and Blomberg [Defense minister], that a war with Poland and France, mean't a war with the UK and probably the USSR. Their fears of the USSR were silenced by the 'non aggression pact' with Stalin, but nothing like that occured with the UK. As far back as 1935 Raeder was trying to convince Hitler of the importance of a navy to fight the RN. By 1937 the Luftwaffe were conducting studies on what it would take to defeat the RAF. Hitler resisted all these efforts forbidding any preperation for war with the UK. Had these prewar efforts to plan for UK invasion as an follow up annex to the invasion of France, then I would put the German chances at over 90%.