Okay, this question has a lot of parts to it and is intensely difficult to gauge. So we'll take it in steps starting with the military of both nations and break it into three parts.

Army: Germany, and no offense to Great Britain, but it really isn't that close. The Wermacht was clearly Britain's superior and remained so for the better part of the entire war. They were their technological betters on the field of battle and their military leadership at the NCO and Officer corps level was outstanding. The one significant drawback was their top level leadership Hitler vs. Churchill, not because he was militarily Hitler's superior, but because he was less likely to interfere and micromanage than Hitler was. Though in fairness that might have been a by product of Hitler's rather unilateral control vs. Churchill's somewhat reigned in power.

Navy: Again, not a close contest, Britain was and remained for the duration of the entire war Germany's undisputed better even including the wolf packs. This is a large oversight on Germany's part prewar, but one that ultimately was never really feasible from the start. The post war restrictions (WWI) on a navy and lack of major port and shipbuilding facilities in the first place meant Germany was years if not decades behind constructing a navy that would be comparable to Britain's let alone achieve dominance of the Channel. Thus Britain gets a decisive if unsurprising nod on this category.

Air Force: This is the close one, in a vacuum the Luftwaffe probably gets the slight nod over the RAF. However, the battle in this case would have been fought over Britain and that means the British possess home field advantage in this air battle. In terms of fuel and rearming, as well as the toll on pilots, the advantages the defense possesses in a battle for air supremacy can not be understated. So, while the Luftwaffe was slightly better overall, the advantages Britain possessed from playing defense would likely very quickly nullify the slight edge the Luftwaffe possesses.

Truthfully, without U.S. support, materially not militarily, purely a supplier rather than an actual combatant, Britain's long term viability is questionable. However, Germany could never have successfully invaded and subdued Great Britain. The difficulties faced by the Allies crossing of D-Day and the very real fear it would fail in spite of their complete naval and air supremacy should demonstrate the enormity of such an undertaking. Germany would have neither of those advantages and in fact be outgunned from a naval standpoint. Assuming for a moment that they could sneak a sizable enough force across the channel to be a threat without being torn to shreds and that they had complete air supremacy so the force could land they still lacked the logistical capability to resupply such an army. That, coupled with the veritable noose the Royal Navy would close around such a force once it had landed and provide close in naval support meant that any such force would be annihilated within days of its landing. Could Germany have forced Britain to capitulate given a few years assuming no outside support was forthcoming, possibly, but that's not a guarantee that such a thing would happen.

Overall and in a total vacuum Germany was the "stronger" nation, but Britain possessed inherent advantages that make it in the best of situations dangerous to invade and without such advantages suicidal. I'm not sure of what qualifies as an "invasion" but only two truly come to mind that succeeded and that's the Roman and Norman invasions, though one could argue William of Orange's 88 usurpation qualifies as an invasion. So in close, no Germany could not have successfully invaded Britain as he had France, but that's hardly fair since France doesn't have a moat.

Britain was a sea power and didn't maintain standing armies of the same scale as the European land powers. It's Home Fleet was stationed at Scapa Flow and one of its tasks was to make any seaborne invasion impractical.

The Home Fleet consisted of 5 battleships, as many heavy cruisers and a swarm of destroyers. In the event of invasion, the Fleet would head into the English Channel and sink the invading army. The RAF would do the same from the air.

At the time there was a lot of popular talk of invasion, but there was no real military concern of a successful large scale invasion. In fact, even during the darkest days Britain was sending its men and equipment overseas for offensive purposes rather than stationing them in Britain for defence against invasion.