Even assuming Hitler overcomes all of Germany's operational disadvantages and successfully invades Great Britain, the resulting insurgency is going to play hell with occupying it and draw resources Germany needs elsewhere. An invasion of Great Britain may be the one thing that draws the USA out of neutrality vis a vis Europe, particularly by 1941 (especially if Japan still decides to attack as per OTL).
I find it hard to believe a British insurgency would be that hard to quell. Despite rhetorics I find it hard to believe that British political leadership or the public would have the will to turn the island into a graveyard. Britain is no Yugoslavia with deep mountain ranges and independent farmers. It was, if not the most then at least one of the most, industrialized societies on Earth, much more developed in many respects than Germany.
What I could see would be similar resistance movement as in France, ie. collection of intelligence, perhaps some quite random terror acts and finally, only when liberation forces were arriving, some armed action.
And of course ultimately, even if UK had a significant insurrection the Wehrmacht and SS would be able to use brutal means to quell it.
The other aspect is the US neutrality. If, instead of Barbarossa Hitler decides to invade Britain during Summer of 1941 a US declaration of war against Germany would have great moral significance, but would it have an effect upon the actual campaign? The amount of troops the US could send would be very small. USAAC wasn't that hot and even the USN carrier air wings were equipped with aircraft inferior to Germans. USN surface ships would be as good as in OTL, though.
But let's say Britain is sufficiently smashed by the end of 1941. What benefits are there for Germany for the summer campaign in 1942 against Soviet Union?
Following come to my mind:
-Trucks, a lot of them, from British civilian and military stock
-Lack of need to provide significant air defence of Germany
-Much merchant shipping for logistics, ability to use Mediterranean and the Black Sea as a supply route
-British industrial capacity. Even if sabotage has a deletorious effect, the industrial capacity could be used to produce consumer goods for German market
-Access (barring US intervention) to the world market for raw materials and export
-Depending upon situation in the Middle East, possibility of using air bases to deal with Soviet oil production, perhaps even Turkish participation which would mean:
a) Easy access through Dardanelles to use Black Sea as logistics route. This would mean the whole logistics setup would change as Germans would have ability to use Ukrainian and Caucasian harbors not only for supplies transported through inland European waterway and railroad routes, but also for goods shipped directly from Germany
b) Ability to strike to the Caucasus from the outset, cutting Soviet oil production
c) Potential use of Caspian Sea for follow-up operations with specially designed light craft
Naturally even a truce would bring much of the same advantages for Germany. But in sum, I think these advantages would turn the tide in favor of Germany.