Yea, that's why I suggested something like "a German military dude looks at the British-French relationship, figures on attacks on France, and works out that or even just decides if they want the UK not to go to war or at least to not be too dangerous they need to present at least a credible threat to the UK's territory itself" even if that's unlikely. The real question to me is "If the UK starts seeing Germany building up to be a threat, how does it react"? I was thinking of diplomatic consequences, especially because thinking Hitler is going to be statisfied with territory in Europe is rather off the table and if they think he's preparing for war with France/the UK then going harder on him might happen*. But what you're saying is interesting and has important bearing-does the UK being prepared enough that the are less fearful of invasion affect public confidence in teh government or willingness to take risks elsewhere. I'm mainly posting because it seems like discussion of this only comes up in relation to sealion and only as a forestalled possibility**. But it seems like a very interesting WI on its own because of how it changes behavior earlier.
RE: your comments on getting distracted from invading France, that would be interesting itself, if they get a bee in their bonnet about invading the UK and then as a result the invasion of France goes badly and it causes...issues.
** "it wouldn't have helped to prepare earlier because the UK would have just upped their own preparations" etc etc.
Which general staff officer is going to go to Hitler in, say, 1938 and say, "Mein fuehrer, I know everybody in my office is hard at work figuring out plans for potential conflict with all of our neighbours, but I've had this brilliant idea that we should temporarily sideline the production of new tanks and pour all that money into building landing ships instead. Yes, I know we won't need them right away, and yes, I know we might need those tanks right away instead, but in the event that one day we successfully conquer France, then we could use them to invade Britain..."
On the diplomatic side, I know everyone likes to blame Chamberlain but I'm not quite sure whether the UK was in a position to be more aggressive diplomatically on its own -- e.g. presumably it won't make France more aggressive, and if France doesn't have their heart in it, the British can't go too far along on their own.
That said, it seems to me that if Britain and France were to push harder earlier, the Germans would be even less prepared for war than they were, and consequently things would go more poorly for them. Historically, the plan that caused France to cave in so quickly was late in the game -- so if, and this is extremely hypothetical, things kick off over the Sudetenland instead of over Poland, France probably wouldn't fall.
And if France doesn't fall then it really doesn't matter how many landing boats you have, you're still not going to invade Britain.
This is why I and others have argued many times that for a German planner it makes rational sense to focus on France, not Britain, and for a German general, it makes rational sense to focus on pushing deeper into France, not completing the defeat of the Allied pocket at Dunkirk. After all, those troops were leaving anyways, and you're going to look awful foolish if in your haste to push them into the sea just that little bit faster, if in doing so you give the French time to reorganize and hold the line.
Just sticking to the production of sea assets, though, the fact is that Britain has a larger stock than Germany at any time from 1918 on and that it can also produce more of them, and faster, than Germany. A naval race would be prohibitively expensive for both sides if it went to extremes, but Britain doesn't have to let Germany build up an advantage in that area and consequently they wouldn't. For Britain control of the seas is the first and most important line of defence, so they're not going to let it go easily.