The figures on troop and cargo movements are in Ruppenthal Logistical Support of the Armies - I think I download my copy from Boston Public Library. The one other constraint is dock capacity, as they could only work daylight hours (no floodlights in wartime).
On troop number, peak monthly arrivals in the UK was 74k in August 1942; this dropped to 53k total for the 6 months November 1942-April 1943, before rising to 174k in November 1943. Certainly getting a million men to the UK by mid 1943 looks possible.
The number of trained divisions is complex, as stripping out cadres for later divisions reduces the training state of the earlier formed divisions. There should be enough trained divisions for a 1943 invasion, although the build up afterwards will be slower.
Shipping capacity is the unknown - as far as anyone can tell there is not an sensible analysis of what shipping might be available if it were not used in the Pacific. Again there is probably enough shipping if Pacific operations were put on hold.
Thank you. That is very interesting. So to summarise:
if we crush the U-boat menace by, say, April 1942, put all troop transports onto the Atlantic run, strip American forces that will arrive too late to support those that will be on time, move Landing ships and landing ship tanks from the Pacific we can get a reasonable force (less than D-day though without Italy the Brits would be stronger too).
Next Question what would the Germans do when they saw this lot coming at them through Calais?