In 1974 the surviving General Staff from the UK + German armies held a war game at Sandhurst to see whether an invasion would have worked, and if it would have succeeded, using the forces that were available to both sides in 1940 it was concluded that:
The Royal Navy would disperse, half to the west of the English Channel, half to the east of the Channel (the german navy was not powerful enough to take on the Royal Navy, then the 2nd largest fleet in the world) and attack any landing force on two sides in the Channel, the RAF would also intervene against the landing craft, and attack the Luftwaffe's aircraft.
Landing beaches on the UK were mined and booby trapped, of the coast at Dover the Army were preparing to pour petrol into the sea and roast the invaders alive. The only place the Germans could land safely and establish a secure beach head was not suitable to deploy tanks/armour.
A series of defensive positions were established by the British across the country where the defenders could fight, then withdraw to the next prepared position and it was agreed by both sides that the Germans would be beaten and would have to evacuate after up to 3 weeks intense fighting.