I would look for the following to make a German victory plausible but by no means guaranteed.
(a) Churchill either dies (car accident in NYC abt 1931?) or is relegated to minor office after failure in the 1920s.
(b) Germans must have some sort of friendly aid on the ground, much more than the limited intelligence network of OTL.
(c) Take out Chain Home. Period.
(d) Remove the British soldiers at Dunkirk from the equation but save their equipment for use in the invasion. Also, minimize British casualties as Dunkirk falls: POWs make better negotiating chips than six figures of war dead.
(e) Consider either modular landing craft that can be built en masse, massive paratrooper drops, or both.
(f) Hammer the infrastructure repeatedly and often to within 5 miles of ones own troops.
(g) At least two feints/distraction landings, something to tie down troops and bugger morale. Land's End, Hull or Grimsby, Newcastle upon Tyne, or Clacton-on-sea would be best. These are for distraction, confusion, and if possible infrastructure destruction wherever possible. Maybe have the Army assign these to SS troops as a 'test' and let them earn their place (while taking disproportionate casualties).
(h) English-speaking troops in British uniforms for further confusion, especially behind the lines. After Dunkirk falls the uniforms and intel will be abundant.
(i) Have a distant but loyal royal in the wings, ideally with a prime minister or chancellor or even restore the office of Protector. Radio and propaganda posters should also be ready to go.