Destroying the Luftwaffe was a huge priority prior to D-Day, and they succeeded. The Luftwaffe was not a factor in the D-Day invasion.
That's true. But there were also plenty of British submarines, destroyers, gunboats, MTBs and other smaller ships. Also Germany didn't have much in the way of specialized maritime attack aircraft (IIRC because Herman Goering didn't want any competition from the German Navy).
The RN certainly would have taken heavy losses from German aircraft, submarines and naval mines (which were planned to be sown both sides of the invasion corridor). But it's hard to imagine that they could have landed enough troops and supplies against a far superior naval force without proper amphibious landing ships. Imagine trying to get tanks and artillery ashore from a barge.
Besides, I never heard anything about the British defenses against an invasion. The D-Day shore defenses were epic, but breached in a day.
But it wasn't on the scale of the 'Atlantic wall'.
Trivia fact. They removed road signs during the war, to make it harder for an invading force to work out where they were. It also made it harder for British people to get around!
They had all sorts of specialised and experimental weaponry, the links below cover some of the details. My late father was involved in their decommissioning as part of his National Service in the very late 1940s.
https://parhamairfieldmuseum.co.uk/british-resistance-organi...
http://www.pillbox-study-group.org.uk/other-wwii-defensive-s...
Given how secret it all was, I wonder how many of their arms and explosives caches are still hidden in the British countryside.
The Korp eventually was overwhelmed because:
1. The British were far better supplied with troops and equipment (also shipped in)
2. The Korp used Enigma, which the British had cracked. With that, the British knew where every German supply ship was and when, and sank most of it. They also knew Rommel's battle plans in advance.
The U-Boot fleet was also lost because of Enigma.