I recently played two games of Spring '42 against my cousin. On BOTH occasions, I captured UK on G2 and he conceded. My cousin would be the first to admit that he is a lousy A&A player, but I still took a lot of joy out of silencing his trash talking, even if only for a moment. It NEVER should have succeeded. It was entirely due to poor decisions on his part. Hobbes is absolutely correct. The UK would have to make GRAVE miscalculations (purchasing and combat), and/or have absolutely horrendous luck with the dice. Hobbes gave it in a nutshell. Let me flesh it out a bit for G1 and G2.
Let’s give Germany the MAXIMUM possible force to bring in for a UK invasion on G1 or G2.
G1 assault on UK: German 1 BOM, 2 FTR, 1 ARM, 1 INF versus UK 2 FTR, 1 ARM, 1 ART, 2 INF, 1 BOM, and the AA. Germany has 4% chance to win with 1 ARM or better (in order to capture, the standard OOL is changed)
G2 assault on UK: Let’s assume on R1, Russia went big into WR alone. Germany has all 6 FTR’s intact. On G1, Germany bought 1 CV, 3 TT, 1 ARM, and the SZ8 SS sank the SZ10 US fleet (50% chance). On G2, Germany could bring as much as 1 BOM, 6 FTR, 5 ARM, 5 INF, 1 BB bombardment shot (though the BB would be better used in SZ 9 to interdict/divert the new US1-built fleet hoping to liberate UK on US2. Let’s keep the BB bombarding UK, however, just to give the best possible odds for the Germans in the invasion of UK. With the sz10 fleet eliminated, the G1 naval purchases are placed in the Baltic rather than the Med. The UK player has to see the threat of a G2 invasion bearing down on them. The UK player, not being COMPLETELY inept, decides to shore up UK. UK purchases 5 INF 3 ARM, the SZ1 TT brought the ECan ARM over to UK. The BB stays put in SZ2 as Germany’s 3 SS, 1 DD, 1 CV have a 98.5% chance of wiping out the BB without needing to divert any air units from the invasion. Germany has 1 BOM, 6 FTR, 5 ARM, 5 INF, and the BB bombardment bearing down on UK, who stands with 7 INF, 1 ART, 5 ARM, 3 FTR, 2 BOM (1 FTR 1 BOM from EUS), 1 AA. Germany’s odds of capturing UK under this optimal condition would still be only 24.5%, and the US would be able to liberate on US2. If Germany parks the BB in sz9 to interdict/divert the US, a liberation by the US would be impossible, but the German odds of success at UK drop to 19% without having the bombardment.
Clearly, a G1 attempt is suicide. Assuming the G1 attack succeeded against the SZ10 fleet (50% chance of success) followed by a best case scenario G2 attempt after a G1 build of 1 CV, 3 TT, 1 ARM (19% chance of success while preventing liberation by US). By compounding the probabilities, your chance of a G2 assault of this sort succeeding is less than 10%. If Germany is missing a single FTR from their original 6, the odds are cut in half again.
IF you are playing against a lousy UK player, you might have better luck forgoing the naval build on G1 altogether. Germany can opt to smash the sz2 fleet, and hope the UK will turn around and buy 1 CV, 2 DD (or a similar navy heavy buy) and ignore the Baltic fleet on UK1 (2 big IF’s), allowing Germany to invade UK with 1 BB shot, 2 loaded transports, 6 FTR, and 1 BOM on G2. 83% chance of success without US units on UK. Long story short, unless you are playing inept opponent(s), a G1 or G2 invasion of UK is a suicide mission.