Could Germany have invaded Great Britain in 1940, or any other time? The answer to that is no, for a number of obvious reasons:

1) They never in their wildest dreams thought that they would ever have to.

2) Because of 1) above they had made no preparations to carry out an amphibious operation on the scale that would be required. It should be noted that even with the unhindered industrial capacity of the US it took the allies four years to mount D-Day - The Dieppe Raid was a test along the lines of the German plan to capture a secondary port intact.

3) After the German invasion and occupation of Norway, Hitler didn't have a navy, an essential for mounting any amphibious operation.

By the bye, Britain would never have "starved to death". If there was one thing that improved out of all recognition during the war it was British agriculture. Even today if Government allowed it and fully supported Britain's farming industry the country could feed itself.