If the Battle of Britain goes against the British, they simply commit the reserves. Even at the peak of the attacks on the southern airfields, the 13 Air Group in Scotland and north England remained as a reserve force in being. The Germans underestimated it precisely once when they attempted a raid on northeast England from Norway; the result was a turkey-shoot. They were unable to neutralize the targeted airfields, and the most-damaged radar station struck during the attack was operational again by the next morning. Perception of the Battle of Britain on both sides both during and after the campaign, I think, tended to suffer from a systematic tendency to underestimate British force and overestimate German force; the Germans believed they could simply snow the British under with superior force, and the British had a bit of a pessimistic air after the fall of all of their continental allies. The British command in this period tended to underestimate the losses the Germans were suffering under the continuous assault, while Goering positively euphorically (not just from the morphine) proclaimed the imminent demise of the RAF at every turn. After the war, of course, everyone adored the story of the plucky underdog fighting the perfidious Nazis to a dead stop.
In addition to that covered by other posters above, the Dutch aren't likely to be long for the peace even if the Germans bypass them again. Even if the Dutch don't enter the war in 1940, they likely still end up in the war by the end of 1941 when the Japanese sail south. An unoccupied and peaceful Netherlands is still almost certain to join the oil embargo against Japan, whether voluntarily or by coercion; the power disparity between the Netherlands and the US is simply too great even with the motherland secure, and everyone with interests in the Far East is both concerned by this point about Japanese ambitions in China and unreasonably optimistic about their willingness to cave under economic pressure. The Germans won't get much in trade due to British interference, and once the Japanese invade for the oil, the Germans will be faced with a independent Dutch base of operations right on their border that's already fighting alongside the British and Americans in the Far East. At that point, assuming it has not already, it will certainly occur to everyone in the OKW that an independent Netherlands is a dagger pointed right at the German heartland so long as it remains an independent co-belligerent alongside the UK against a German ally.
Same with Franco; Kovax is precisely correct there. Franco was still fighting Republican resistance fighters for a decade after the civil war officially ended. He was massively reliant on food imports from Portugal and Argentina to keep the population fed; even with this support, something like 200 thousand people died between 1939 and 1945 as I recall. Even this line of food and other supplies such as fuel would have been cut short had he entered the war formally. The Spanish army simply could not be maintained under arms for any longer; he immediately cut it down to a quarter of its size, and even the second expansion after the Fall of France was constrained by hard financial limits. The ludicrous demands Franco made as the price for his entry into the war were, if anything, too modest. Germany simply could not afford to feed and arm all of Spain, especially when it was busy feeding and arming itself; Spain only offered a far larger point of vulnerability.
Similar problems hold for coastal Anatolia. British naval superiority in the Eastern Mediterranean and Turkish hostility (unless you're presupposing they join the Axis somehow, in which case we would likely end up with a second Yugoslavia where those who sign the pact return to find themselves no longer in power) will make any "quick zip" neither quick nor zippy. Infrastructure is problematic, and the major problem with the Turkish army of the period, a lack of arms for the soldiers, will be quickly rectified by Lend-Lease sailing right up the Persian Gulf and up the Tigris and Euphrates to eastern Anatolia. While the British won't be able to save Istanbul or the Aegean coast, they can move up forces from India (just as they would do against the 1941 coup) and the Levant to oppose the Germans in the rough terrain of the east. The necessary diversion of forces to the Middle East in order to conquer not only Turkey, but everything as far as Suez also gives the Soviets more time to repair the damage caused by the Great Purges in their NCO ranks, a fear that Hitler of all people is rather keen to avoid. A quick sprint along the coast that bypasses the Anatolian Highlands is perhaps even worse than a full-in invasion to subdue all of Turkey: it gains little in the way of reduced manpower requirements and makes the entire supply line along the coastal strip vulnerable to raids both from the interior and from the coast.
The Italian Regia Marina, I think, you give too little credit. The Battle of Taranto entirely failed in its primary aim to cut Italian convoys across the Mediterranean. The Regia Marina did not lose the majority of their operating force in the opening shots. The heaviest losses at Taranto and the only disproportionate losses as a proportion of the total fleet were in the battleships, of which only one (Conte di Cavour) was out of commission for more than half a year. Taranto bought the British precisely half a year of operational freedom, but the Regia Marina continued to operate aggressively and thus kept the Mediterranean contested until fuel shortages, rather than any sort of decisive battle, shut down most of their offensive operations. A few torpedo nets at Taranto will not save the Regia Marina from a slow and painful death; only massive infusions of fuel will do that.
