If we assume a time period approximate to June 18th, 'Britain's finest hour', then certainly, Britain's position was not an enviable one, but defeat was not a foregone conclusion. In the eyes of the world at least, especially the US, Britain's survival seemed dependent on its ability to resist the inexorable German invasion. After all, the panzers had rolled across Western Europe in little over a month, Operation Sealion was surely mere weeks away? Such viewpoints were popularised largely by propaganda, though not at all helped in the popular narrative by speeches such as this.

The Netherlands, France, Belgium and Luxembourg had all capitulated, with the French armistice coming into effect by June 16th. At home, the British Army, now with returned units from France and newly raised units, consisted of around 22 divisions at roughly half strength, with more still mobilizing. These units lacked much of their light and heavy artillery, motor transport and critically, ammunition - much of it having been left behind. It is estimated (generously), that Britain possessed around 500 tanks at this time, most of them in the light to medium 'cruiser' variety, and around 800 medium to heavy artillery pieces. The Local Defence Volunteers had been founded in early May and by June consisted of around 1.5 million volunteers whom, like their professional counterparts, were woefully under equipped, but in good morale.

In the naval theatre, the loss of the French navy and their Atlantic bases was a body blow to the Royal Navy. Prien's U-Boats now had access to the Atlantic, and enjoyed their first 'happy time', sinking around 270 ships before October. The loss of 10 destroyers in the Norwegian and French campaigns, and a further 10 in the Atlantic compromised an effective naval response to invasion, and to policing its convoys (though this was remedied by the Destroyers for Bases Agreement). Compounded with the Italian entry into the war, the RN was now stretched in its commitments from the Atlantic to the Pacific Ocean. In the Mediterranean it had to guard British possessions from Gibraltar to Palestine, as well as support Greece and the army in Egypt from a modern and effective Regia Marina.

This being said, the Royal Navy still effectively dwarfed both the Kriegsmarine and the Regia Marina. For any effective invasion operation to be undertaken, Germany would need clear naval superiority, something which it could not accomplish against the Royal Navy's Home Fleet. The Kriegsmarine had lost 3 cruisers and 10 destroyers in the Norwegian campaign, which would have provided the bulk of any invasion fleet. While the Royal Navy had taken similar losses, these were easily absorbed and were soon to be rapidly replaced with a massive expansion program begun in 1938. German naval losses had also disenfranchised Hitler to the need for a large surface fleet, and despite Prien's objections, the Kriegsmarine's surface units, including its precious battleships, would lose their limelight to the u-boats.

Despite the desperation that would be seen in the Battle of Britain, Britain remained strong in its air assets. The loss of 106 aircraft covering the Dunkirk evacuation was quickly replaced with Britain's steadily increasing industrial output. Around 750 single-seat fighter aircraft were available to Fighter Command at this time, the majority the sturdy Hawker Hurricane, but the remainder being the superb Supermarine Spitfire. Most concerning however were the losses in pilots. Most RAF pilots lacked the experience of their Luftwaffe counterparts, being quickly trained, though their officers had often seen action over France, and were helped by significant contributions of pilots from the Commonwealth and other Allies. Fuel stocks had been expanded in the prelude to hostilities, as had the number of airfields. The RAF's Dowding System - of Chain Home Radar stations and a hierarchy of reporting stations, providing undoubtedly the finest air-interception system in the world at the time.

Overall, Britain's defending forces both navally and aeronautically were well equipped to deal with German invasion, but limited in any offensive operations, while the British army was nowhere near equipped to handle a German attack until at least the following year. However, opposing German forces are often over-exaggerated in their ability to affect an invasion or destroy Britain's supply lines.

The Luftwaffe's air forces ranged against the UK consisted of around 2500 aircraft; 800 single-engine and 300 twin-engine fighters, 1000 medium bombers and 300 dive-bombers. Germain air-crews gained valuable experience in the Spanish civil-war, as well as over France and the Low Countries. Their primary aircraft consisted of the capable Dornier Do17 and Heinkel HE111 medium bombers, and the excellent Messerschmidt Me-109. These fighters would however, be substantially hindered by their short operational range, possessing perhaps 20 minutes worth of fuel for operations over Britain. The early successes enjoyed by the Luftwaffe were largely the result of it being a 'tactical' air force, designed for operations in support of the army. Certainly the stunned allied troops in Belgium could attest to its awesome capacity as aerial artillery, however the need for deep-strikes against British industrial centres and distant RAF airfields revealed key weaknesses in the Luftwaffe's capacity for strategic operation. Moreover, the Luftwaffe's hitherto poor record against warships precluded it from effectively disrupting RN operations, and would mean in any amphibious operation, it would remain dangerously overstretched.

The requirements to be placed on the German Army for any attempt at crossing the channel were also far in excess of its capabilities. Army High Command (OKW) had originally called for a landing of 40 divisions, which was ultimately revised to a still unrealistic 9 divisions. (By comparison, the Normandy landings consisted of 8). No doubt the German Army, with its emphasis on mobility and armoured penetration, and fighting a vastly under-equipped British Army, would have been able to secure substantial bridgeheads had it been landed, however it is likely that its Blitzkrieg style assault may well have faltered upon encountering as dense an urban area as London. London of course, the largest city in the world at this time, would have made dangerous ground for tanks, and German infantry would be slowed by extensive house-to-house operations against a familiar and fanatical defender. German airborne forces were recognised as a significant risk by British High Command, especially following the daring assaults on Eben Emael and Rotterdam. However the saturation of LDV units across the countryside would have hindered attempts at airborne link-ups, or at the very least, forbade paratroopers any hope of landing stealthily.

Perhaps the most critical factor in determining the success of an invasion however, was the ability of the Kriegsmarine to first transport and then supply such a force. As detailed above, the surface escorts of the German navy were no where near numerous enough to support an amphibious landing, especially not one conducted in the face of the Home fleet, which would be forced to 'unerringly commit its naval forces'. Despite Admiral Donitz's guarantees of being able to keep the RN at bay, a mass, albeit piecemeal, assault by the RNs largest units, as well as the 500 or MTBs hidden along the south coast, would likely result in slaughter of the hapless embarked infantry, and the marooning of any that had disembarked. Not only this, but the Kriegsmarine was desperately lacking amphibious craft, especially for an operation of such magnitude. The 2,400 requisitioned barges from Dutch and Belgian canals were woefully inadequate for a channel crossing, and the specialised craft far too few in number. Donitz would be forced to settle for his protracted u-boat campaign to force Britain from the war.

To conclude then, Britain's position in June 1940 was poor. The Royal Navy was overstretched, its air force was small, and the army desperately poorly equipped. It stood no chance of waging an offensive war in Western Europe without substantial help. However, the commonly held views that invasion was imminent (that it was saved by the 'divine providence of the few') and Britain was completely vulnerable to it, are misconceptions. The weaknesses of the Home Fleet and the RAF, and the capabilities of their German counterparts to adequately support an invasion are grossly over exaggerated. While the u-boat campaign would ultimately evolve to become a very significant threat, arguably the greatest the country has faced, in June 1940 Britain was certainly very far from 'screwed'.

^(Please be kind this is my first response on this) ^(sub)^(.)