let us assume that the armistice was signed in September 1940 and Barbarossa goes off in early June 1941 a little ahead of OTL.
1. There may be some PoWs still in German hands, certainly the most valuable ones such as senior officers, pilots, etc would be the last to come home. Even if all are home by now many were demobilized and would not be happy about being recalled.
2. Britain is in bad financial shape - expenditures and "cash and carry" from September 1939 through September 1940 have drained cash reserves, and there were necessary expenses of rebuilding bombing damage and air defenses which would be a major priority, as well as rebuilding the RN losses. North Africa/Egypt and the Far East have needed resources as well.
3. As pointed out, blockading France would mean war with Vichy, absent that blockade Germany can still access the world, of course shipments through Italy will be there as well.
4. No matter what Roosevelt might want to do, there is no way the US Congress (and population) will support the UK going back to war with Germany to "save" the USSR. For much of the US Stalin was not much better (if at all) than Hitler.
In this scenario Britain in June, 1941 is much weaker than June, 1941 OTL and is not going to have the USA giving much help. OTL (and probably ITTL) the USA was in a military buildup which was fueled by worries about Japan as much or more so than Germany. There were voices raised against sending much to the UK when US troops were short of basics. And what can the UK do? They can try and send supplies to the USSR but losses will be significant enroute, and they can't replace shipping losses with "Liberty" ships here. Also, how do they pay for the supplies they send...will the Soviets pay in cash (stolen Spanish gold perhaps) or will the average Britisher decide they want more austerity to support the USSR (I doubt it). They can attack German bases in France, Holland, Belgium, Norway but especially in France (and perhaps Norway as well) the "local" military will respond against these raids (sea or air). Because the RAF will have had to concentrate more on air defense/fighters with British/Commonwealth construction, and no US aircraft to speak of, resource limitations means Bomber Command ITTL will be much smaller than 1941 OTL. The means to do deep attacks/attacks on Germany even as nuisance raids will be quite limited.
If you make the assumption that things went badly enough for the UK to accept a gentle armistice in fall of 1940, then the UK of summer 1941 absent LL and strong US support simply is in no condition to go back to war with Germany without cutting its own throat. IMHO the reason the USA supported the USSR in 1941 was because the USSR was seen as being an ally to "plucky Britain", so one swallowed any misgivings about Stalin. That would not be the case if Britain went back to war to try and save Stalin.
1. There may be some PoWs still in German hands, certainly the most valuable ones such as senior officers, pilots, etc would be the last to come home. Even if all are home by now many were demobilized and would not be happy about being recalled.
2. Britain is in bad financial shape - expenditures and "cash and carry" from September 1939 through September 1940 have drained cash reserves, and there were necessary expenses of rebuilding bombing damage and air defenses which would be a major priority, as well as rebuilding the RN losses. North Africa/Egypt and the Far East have needed resources as well.
3. As pointed out, blockading France would mean war with Vichy, absent that blockade Germany can still access the world, of course shipments through Italy will be there as well.
4. No matter what Roosevelt might want to do, there is no way the US Congress (and population) will support the UK going back to war with Germany to "save" the USSR. For much of the US Stalin was not much better (if at all) than Hitler.
In this scenario Britain in June, 1941 is much weaker than June, 1941 OTL and is not going to have the USA giving much help. OTL (and probably ITTL) the USA was in a military buildup which was fueled by worries about Japan as much or more so than Germany. There were voices raised against sending much to the UK when US troops were short of basics. And what can the UK do? They can try and send supplies to the USSR but losses will be significant enroute, and they can't replace shipping losses with "Liberty" ships here. Also, how do they pay for the supplies they send...will the Soviets pay in cash (stolen Spanish gold perhaps) or will the average Britisher decide they want more austerity to support the USSR (I doubt it). They can attack German bases in France, Holland, Belgium, Norway but especially in France (and perhaps Norway as well) the "local" military will respond against these raids (sea or air). Because the RAF will have had to concentrate more on air defense/fighters with British/Commonwealth construction, and no US aircraft to speak of, resource limitations means Bomber Command ITTL will be much smaller than 1941 OTL. The means to do deep attacks/attacks on Germany even as nuisance raids will be quite limited.
If you make the assumption that things went badly enough for the UK to accept a gentle armistice in fall of 1940, then the UK of summer 1941 absent LL and strong US support simply is in no condition to go back to war with Germany without cutting its own throat. IMHO the reason the USA supported the USSR in 1941 was because the USSR was seen as being an ally to "plucky Britain", so one swallowed any misgivings about Stalin. That would not be the case if Britain went back to war to try and save Stalin.