If Germany does Sealion. Then you get as many infantry as possible with your money, you bring things from Canada. Land all planes possible. And save any kind of navy you will still have so that it can haunt Germany later on.
The purpose of this is to make London easier for America to liberate and take as many German things with you to the grave as possible. This makes it harder to defend England as Germany has to ferry infantry. (The minor Ic can build only minimal defense) and a more difficult attack on the Soviet Union.
In my games sometimes Germany gets hit so hard by taking London that the Soviet Union moves into the grey spaces and gets money. My latest game they actually made an attack on Berlin.
In Africa once the minor IC in South Africa stops getting money it gets pretty impossible to hold on to. Just keep your forces in Anglo Egypt because Italy almost always has to go through their to get to the big money. To solve this problem I usually send Indians to Africa to help with the Italy threat and more importantly keep them out of the Middle East where there is more cash and the opportunity to attack the Soviet Union from another side.
Germany goes Barbarossa. Feed the South Africa IC and keep Italy out of Africa. Don’t let them get that factory in Egypt or even worse Iraq. If you can get enough man power in Africa then you can go through the Middle East and up into the Soviet Union to assist them from Germany who by now is probably inches from Moscow.
Coordinate with the United States and get a big enough navy to open up a Western front. This is the most obvious way to help the Soviet Union. You can even send some spare air force to the Soviet Union if possible.