This would be a fantastic tactic to use in a game, if only it allowed for Germany to spend their IPC’s accordingly on a proper Sea Lion invasion.
To put simply, the UK reacts to whatever Germany does. If Germany goes for London, then the UK prioritizes London. If Germany goes for Africa, then the UK prioritizes Africa.
All it would take for me to get you to pull out EVERY single unit from London would be to employ a fake Sealion attack of mobilizing the classic Sealion build of an aircraft carrier and 2 transports, because this build screams Sealion at the UK player, I’ve pretty much just told you I’m going to do Sealion. You pull out of London without Germany even having spent the 49 IPC’s on 7 transports to bring units onto London, only having to take 3 transports worth of units to secure London and Scotland, not spend anything to do it, and keep all their said units alive for Barbarossa and potentially Afrika Korps.
This wouldn’t be cost effective even the slightest, the British really can’t afford to hand London to Germany like this because frankly, even if you get the German player to spend those 49 IPC’s on a bunch of transports they won’t end up using, the whole key factor of defending London is to make sure Germany at the very least has to fight for it, giving up something to compensate for another factor whilst not taking away from the preexisting German army just is not a cost-effective method, each player of the Allied team should be doing absolutely everything in their power to whittle away the German War Machine as much as possible. It’s their duty to make sure that whatever strategy the German player attempts to employ that the Allies should make damn well assure that Germany costs dearly in another aspect to which they did not do.
That’s the whole idea of Sealion, when Germany sends 10 infantry and 10 armory and a crap ton of the Luftwaffe to the British Isles they are doing so at the cost of really setting themselves back with the inevitable invasion of the Soviet Union, let alone the fact that the Soviet Union will be put in the position of going on the offensive.
Just my thoughts though, personally I’m not one to give something up without a fight as the Allied player, since being able to cut away from the inevitable Axis success is always the #1 priority as of early-mid game.