Grimm
If Britain made peace then Germany is in a markedly better position. less the relatively small number of extra troops than the freeing up of air power, the fact Germany is no longer blockaded and can import raw materials [provided they can pay for them

] and the loss of destruction to production. However there are a number of other factors.
a) As archaeogeek says if Germany tries going earlier they get bogged down in the mud.
b) It is highly unlikely Stalin would be caught by surprise if Britain has made peace so the huge losses the Russians faced in the 1st days of the OTL invasion are likely to be significantly reduced.
c) Hitler might well feel even more over-confident and hence, moblise even less, not be particularly interesting in getting more allies, especially from places like Italy, which would mean an additional burden in organising, moving and supplying such units.
d) Possibly most of all if the Germans get a bit further they could become fatally over-stretched in 41. Say if instead of last gasp efforts reaching the outskirts of Moscow the Germans are in a position, at the start of November, to launch a full scale attack? Moscow is already well fortified and has a large population that could be moblised. It's in the centre of the Russian transport grid and also politically important. As such neither side would be that willing to give up on the fight. Can you imagine an already over-stretched Army Group Centre engaged in bitter street to street fighting, with Hitler ordering them to keeping attacking, when the winter really hits and then shortly afterwards the Siberians strike what's left of their flanks.
There is the danger that seeing itself alone and with the extra forces Germany can commit, that the SU might just collapse. Especially with the mental collapse that Stalin seemed to have suffered at the time. Or that he does something really stupid like try to attack 1st. However I think either of those are pretty unlikely.
In the longer term, without the other fronts the west supplied or the supplies that helped the SU it will be markedly worse off. However this will mainly affect it's ability to counter-attack rapidly and make the deep advances it did OTL from 43 onwards. Hence there is likely to be a long and gruelling slog with both sides battering away at each other.
In terms of other areas I'm not so sure that Italy would be that closely involved. I could see Britain fairly easily driving a wedge between the two at the peace talks. Making clear that while they accept the Germans defeated them and offering acceptance of their gains in Europe and even return of lost pre-WWI colonies, that the back-stab by the Italians will
not be rewarded. Basically Hitler wants a clear line for an attack on Russia. He also wants stable relations with Britain and that will be more important to him than boosting the ego of Mussolini. Hence I could see Italy getting few if any gains and Mussolini being very frustrated. He is hence unlikely to support major forces going into Russia. He may have a hack at Greece but Britain can again help the Greeks without committing major forces - seeking to avoid a clash with Germany over the Rumanian oil-fields.
Steve
stevep, while Stalin may indeed be more alert if Hitler is not fighting the British this does not free up forces for the USSR but it does free up substantial German and Italian forces from the Balkans, plus no Afrika Korps is needed.
The coup in Yugoslavia certainly isn't coming off if the British aren't even in the war and this may even add a division or two in volunteers to Hitler's forces, plus freeing up substantial garrison forces.
Greece may well seek Hitler's aid to negotiate a settlement with Mussolini as British help will not be forthcoming.
On top of that Italy can now spare a few more units on the Eastern Front and Hitler can run down the western garrisons a bit more.
Overall we would be talking several dozen new divisions for Barbarossa, which did such harm to the USSR OTL. Not enough to finish the USSR in a single campaign but isolating Leningrad completely or even taking Moscow would seem within reach.
On France Hitler didn't totally disarm France OTL. Vichy was allowed 100K in the unoccupied zone and had close to half a million men in the colonies, if rather lacking in firepower and modern equipment. With the British coming to terms he may expand the unoccupied zone and permit a modest French army expanion in the unoccupied zone. 300K wouldn't be a threat to Germany and would give the French some hope for better things in the future, especially if some territory was returned also.