For a successful Sealion the Germans need to do many things. The first of which is to throw out the window their OTL plans.
Now, starting from scratch...
More U-Boats! Like 200 of them. Once WWII starts they deploy to blockade Britain, cutting her resources, supplies, & so forth as much as possible.
The Luftwaffe develops armour piercing bombs, air dropped torpedos, & several dedicated anti-ship squadrons.
The Luftwaffe must also continue to concentrate their efforts against the RAF, & other important military targets, during the Battle of Britain whilst somewhat ignoring the temptation to blast pointlessly away at London & so forth.
The Kriegsmarine learns from its disasterous Norway campaign & drops any idea of simply sailing into Dover harbour. Instead they learn how to fight a real naval battle. Plus they make sure all their surface ships are ready for Sealion regardless.
The planners for Sealion realise that landing at the White Cliffs of Dover, & there abouts, are just going to get them slaughtered. So they look for a nice sized port, which has lovely beaches nearby, for an amphibious landing. That means Ramsgate.
So essentially the Germans pull a Normandy on the British, by landing at the beaches just to the south of Ramsgate, whilst spreading disinformation that the landings will be at Dover (ie their OTL plans). The landings, though, still consist of several hundred so-called "Rhine Barges", as well as four old large merchantmen, which are deliberately beached as ad hoc landing ships (note the infamous "Rhine Barges" are only used in the initial wave, not afterwards).
Along with this their two airborne divisions land some 5-10kms inland to act as the first line of defence, whilst a light mechanised/motorised force, of about a division in size, races to capture the port at Ramsgate, whilst the other divisions dig in to secure the beachhead & be ready to support the airborne divisions if need be. Meanwhile dozens of Luftwaffe fighter squadrons fly CAP to keep the RAF away.
Whilst all this is going on, the sizeable German E-Boat fleet keeps the RN destroyers & cruisers away from the second & subsequent waves of German reinforcements coming in on Baltic Sea passenger & vehicle ferries, not to mention other merchantmen.
The main Kriegsmarine battlefleet, considering of Bismarck, the two Scharnhorst class ships, the two Schlesien class ships, & the two surviving pocket battleships, supported by cruisers & destroyers, lay in wait in the English Channel for the inevitable clash with the RN Home Fleet. Whilst the southern approches to the English Channel are mined, closing them to any RN Fleet, the northern approach simply can't be due to the size. Here the Kriegsmarine station something like 25 U-Boats in the North Sea in anticipation of the Home Fleet's movements. Similarly those dedicated anti-ship Luftwaffe squadrons are also ready to pounce upon the Home Fleet once it enters the North Sea. In doing so, the combined efforts of the U-Boats & Luftwaffe should see at least a third of the Home Fleet put out of commission.
The forthcoming naval battle, in the northern approaches to the English Channel, should thus be a somewhat balanced affair. The RN, however, loses even though the Germans suffer high casualties as well. This naval battle outcome, though, is just enough for the Germans to keep their convoy routes open to reinforce & supply their invasion army which, within a few months, has defeated the British.
Hitler then dines at Buckingham Palace, not long after Britain's capitulation, which he promptly orders to be destroyed the next day.
