Hitler never had a serious intention of conquering Great Britain - he wrote already in Mein Kampf that a downfall of the Empire would not benefit Germany which had no interest in overseas colonies (the Lebensraum was east ...), but only the comparative "upstarts" Japan and the USA which would then simply assume control in the void left by the Empire. He also regarded the British "race" as largely "Aryan" (also in Mein Kampf), albeit unfortunately infested with the dark powers of "world jewry".
Dunkirk was an extended hand to the UK to enter into armistice talks with full honor intact. Hitler did not want an England too weak for fear of a crumbling Empire. When that failed, there were no serious plans in the cupboard to invade and occupy England - neither the German Army nor the Kriegsmarine nor the Luftwaffe were geared for a large scale invasion from the sea, they were forces created solely for and to support land wars. Rather Hitler believed that the U-Boat embargo and the London Blitz would bow GB into submission and once again into the entering into peace talks, effectively ceding Continental Europe to the Third Reich and leaving the Empire unscathed. It didn't work that way, but Churchill saw the danger that it could and he farsightedly kept England long enough in the war unconquered for Japan to attack the US and the US then enter WW II.
The Third Reich, for all its inherent evilness, did not even bother to occupy France in full, much less would it have occupied the Brit Isles which are - let's face it - strategically irrelevant for control of the Continent. Well, at least we had the Channel Islands which you guys, adding insult to injury, did not even reconquer after D-Day but simply circumvented until Germany capitulated.
