Sealion was pretty much an impossible goal.
First, the Germans had absolutely NO way of getting troops across the channel. Their plan was to try and use barges, river barges, to ship troops across the channel. First off, the number of troops that would be needed to even take and hold the beach would've been enormous in number; the German plan hoped to get 16 divisions across in the first month.
This is all assuming, of course, that they Germans could even get across the channel. The most obvious barrier to this is the Royal Navy. The British Home Fleet outnumbered German surface vessels by at least ten to one in the Fall of 1940, and there were another five British battleships in production at the time; two entered service before the year was out.
The argument is sometimes made that "massive airpower could keep the RN out of the channel long enough to get troops across." First, you must realize that this is complete hogwash. The ineffectiveness of German pilots against ships was demonstrated effectively during the Dunkirk evacuation; though there were thousands of dive bombers strafing and dive-bombing the British ships, only one was sunk, of the many hundreds there.
However, assuming that somehow, some way, the Germans were able to keep the Royal Navy at bay, it's quite likely that the flotilla would have foundered in the channel itself. As I said before, the Germans had no landing ships, they intended to use confiscated river barges to ship men and materiel across. I don't know how familiar you are with the Channel's weather, but it's pretty much never favorable, and it's never, ever calm-watered. But again, assuming the Germans managed to get across the channel, how many men were they going to land? As I noted above, they hoped to get 16 divisions across in the first month. The standing British Army in September 1940 was at least of equal size, including two new armored divisions, of which creation was begun on the German model during Fall WeiB. In addition, the British were fully prepared to use poison gas on the beaches to defend the home island, if the Germans ever landed.
As if the death stroke had not already been dealt, there was no way the Luftwaffe was going to beat the RAF, either. British plane production already outpaced German production in 1940, and most of them were the new Spitfires, which put the RAF on a more equal footing with the BF-109s. Further to the point, however, is that the entire RAF was not engaged in the Battle of Britain; only the bare minimum needed to stave the Germans off was sent southeast; the rest was kept at airbases in the Midlands, out of the reach of German fighters. Granted, their bombers could reach that far, but only sheer madness would precede an unescorted bomber raid against an airfield; perhaps their only chance would be at night, but then there's a quite high chance that the bombers might never find nor hit their intended targets in the first place.
Had the Germans kept up the bombing of the RAF airfields, rather than switching to the cities as they did, the RAF would have simply withdrawn to the Midlands airfields to regroup long before it was truly "beaten" in the South. Because of this capability, the Luftwaffe really had no chance of ever besting the RAF permanently, and thus, we can deduce that RAF air cover would be present during any German attempt to "scare off" the Royal Navy, or during any beach landing.
So in conclusion, Sea Lion couldn't have been successful. It's not really a what if, it's all but fact; unless you journey into lands of the truly fantastic.