Thumpalumpacus
Major
I would want sealion to go ahead because the RAN would send the invasion fleet to the bottom of the channel[...]
Presumably using the Luftwaffe bombers to sink those ships, meaning that the German troops already landed would have much less on-call air support. Those LW bombers would need escort, too, meaning less CAP for the barges (barges!) bringing across resupply, meaning that even Hampdens and Blenheims would have a decent chance of cutting landed Germans off from resupply.
Pushing Sealion forward without air supremacy would be pushing oneself onto a knight's fork, because with the KM so hurt after Norway, the LW would be forced to choose between defending barges, supporting ground troops, or attacking the RN. It couldn't do even two of those at once. It simply didn't have the strength.