'PE' was the British overall designation of the defensive operations which were to be undertaken by warships in the event that the Germans launched their 'Seelöwe' invasion of the UK (5 July 1940 onward).

It was at the end of May, even while 'Dynamo' was taking place, that the Admiralty sent to the commanders-in-chief of all the commands in home waters a directive which outlined its anti-invasion strategy. In the belief that the Germans would opt for the shortest possible crossing and be prepared to accept very severe losses in order to land German army formations in England, the Admiralty stressed the importance of attacking the German invasion forces before they departed the harbours of occupied Europe, a tactic which required the earliest possible indication of the German assembly by British intelligence and reconnaissance assets. When the assembly of an invasion fleet had thus been detected, it was to be bombed, mined and shelled in its anchorages. However, should it prove impossible to destroy the invasion fleet before it put to sea, it was then to be attacked at the point of arrival. As this latter could not be known in advance, the Admiralty judged that the British forces had necessarily to be disposed to cover the entire south-eastern corner of England between the Wash in the north-eat and Newhaven in the south-west. The directive then considered a possible third eventuality of the German invasion force being spotted on its way across to England, making it possible for the Royal Navy to intercept the invasion on passage.

The Admiralty reckoned that to the destruction of the German invasion fleet at sea would require the use of four destroyer flotillas (36 ships), with additional firepower provided by cruisers. The destroyer flotillas were to be based in the Humber, Harwich, Sheerness and either Dover or Portsmouth, where they would be well placed to sortie and fall on the German invasion force as it reached the English coast, regardless of the Germans' exact landfall. The Admiralty also proposed that the largest number possible of destroyers, escort vessels, corvettes and other vessels as could be spared from escort duties should be allotted to the possible invasion area. Finally, the Admiralty added, small craft were to be collected immediately to maintain a watch close inshore and in order to hamper the German operations.

The Admiralty believed that the passage of the invasion force might also be accompanied by a diversionary sortie of the fast battle-cruisers Scharnhorst and Gneisenau in northern waters, and perhaps by a sortie into the southern part of the North Sea by the two old pre-Dreadnought battleships Schlesien and Schleswig-Holstein accompanied by as many as five cruisers. Against any such eventuality the Royal Navy had to have available a sufficiency of capital ships (battleships, battle-cruisers and aircraft carriers) and cruisers to deal with the threats so posed.

Comprehensive though these plans were, they committed almost half of the UK’s remaining destroyer strength to a defensive screen dispersed round England’s south-eastern shores, so inevitably denuding Admiral Sir Martin Dunbar-Nasmith’s Western Approaches Command, which was responsible for the Atlantic convoys, and Admiral Sir Charles Forbes’s Home Fleet. By the end of July there were 32 destroyers and five corvettes in Admiral the Hon. Sir Reginald Plunkett-Ernle-Erle-Drax’s Nore Command alone. Moreover, six cruisers were also removed from the Home Fleet, which already had a shortage of this type of warship, and similarly dispersed to south-eastern naval bases. As early as 17 May the Admiralty (in essence Admiral Sir Dudley Pound, the First Sea Lord) had suggested to Forbes that the Home Fleet’s battleships should also come south to Plymouth. At the other end of the scale, anti-submarine trawlers were taken away from the escort duties, in which the were invaluable, and despatched to supplement the new Auxiliary Patrol of about 1,000 armed trawlers and drifters, of which about a third were always patrolling on watch for the invasion fleet.

Forbes strongly opposed this drain on the strength of his fleet and the concomitant reduction in the level of protection for merchant shipping on the grounds, he believed, that an attempted invasion was impossible unless and until Reichsmarschall Hermann Göring’s Luftwaffe had decisively defeated Air Chief Marshal Sir Hugh Dowding’s Fighter Command over the English Channel and southern England. It was Forbes’s opinion, therefore, that the defeat of Fighter Command, should this happen, would be the right moment to redeploy the Home Fleet for a last battle to prevent a German invasion. In May Forbes was successful in resisting a proposal that the Home Fleet should be rebased to Plymouth, and in July he just as vehemently opposed a suggestion that he should detach two of his battleships to Liverpool. Forbes did agree, however, that if indications of a German invasion across the North Sea became evident, he would transfer the Home Fleet from Scapa Flow in the Orkney islands group south to Rosyth in the Firth of Forth.

The dispute was finally resolved on 20 July when an Admiralty directive laid down that British heavy ships should not move into the southern part of the North Sea unless German capital ships were being used to cover an invasion force, in which event the British heavy ships were to engage the the invasion force at the earliest possible opportunity.

Needless to say, the commanders-in-chief whose commands lay in the path of the possible invasion routes (Admiral Sir William James’s Portsmouth Command, Vice Admiral Sir Bertram Ramsay’s Dover Command and Plunkett’s Nore Command) saw matters in a different light, and wished to be allocated and to retain all the cruisers and destroyers which the Admiralty sent them. As Plunkett put it, 'to destroy an invading force we need gunfire and plenty of it'.

The crux of the entire matter of the Royal Navy’s optimum deployment against an invasion force was the degree of advanced warning which could be provided. 'Dynamo' had shown that reinforcements could reach Dover from Rosyth within 24 hours. Even ships escorting convoys in the Western Approaches could be recalled within a few days.