I think the #1 lesson I learned on the forums and actually playing the game is this.
UK1 YOU ALWAYS BUILD: 6 INF + 1FTR or 9 INF on London.
If Germany is going to go ahead with sea lion make it a honest attempt. Defend London to the best of your ability on UK1 and UK2 and see what happens.
UK gets in trouble when they do not defend London to the max on their first 2 turns and goes off script. UK cannot afford to miscalculate the situation and be under manned on the first 2 turns on London and let Germany waltz in and take it.
UK2 Your purchase is a response to Germany. If they go Sea Lion build you MUST build more defense on the island. If they go with a Russia build and no TRS that is when you can build that minor IC somewhere and start looking at areas to annoy the Axis.
