Out of maybe 15 "serious" games I've played or watched, I've seen 2 sealions. I've also seen one "cheapshot" which ended the game in 1940. Both of them succeeded overrunning most of the UK.
First one though, lost anyway. Russians entered the war, and so did the Americans, early, and while the UK was badly battered, there was an enormous amount of troops in the Home island, and the supply lines got cut, and the cream of the German army got pulverized.
Second game, kind of semi classic. Germans took France down a bit earlier than usual, went into Spain, took Gibraltar, let the Italians out, conquered the UK in about 1941, and turned around and did a late 42 Barb. Won quite handily.
#3 was a bit of an opportunistic thing. CW went for the "Big BEF" and advance built Alexander, and rushed him and a whole bunch of stuff into France. I think they had like 1 corps and 1 gun remaining in the whole UK, so the Germans didn't even go into France proper (they took out Belgium), and just pell mell rushed in with the para, and Bock on an Amph, and then got a few more bodies in the subsequent impulses, and wound up overrunning the country by the summer of 40.