If the fate of Wellington should be relevant, it would only be bacause the British are defeated in Spain so early, that substantial French reinforcements from Spain can reach Saxony in time to inflict a decisive victory on the allies there. Without a (very) decdisive voictory over the allies in central Europe - there will be no French victory in the war - no matter what happens in Spain or with Wellington.
The most certain way to ensure a French victory in Saxony, would be letting the Austrians stay neutral. In 1813 they provided the biggest contintgent to the allied army and also its very competent leadership through Schwarzenberg (the Eisenhower of Nap.Wars) and Radetzky (the mastermind of the succesful allied grand plan).
A decisive British defeat in Spain in early 1813 could contribute too the Austrians staying neutral, but a more flexible French diplomacy, like in promising the Austrians their lost territories back (but not the HRE), would have been more important in this context.
OTL Leipzig could IMHO very well have been won by the French. If say Schwarzenberg had obliged to the Zar's demand of no troops west of the Pleisse, then at least two more French Armycorps (incl Young Guard) would have been available in the decisive central south sector (in OTL they were diverted to engage the Austrian III and IV AC sent west of Pleisse). Where more allied troops in the central south sector probably only would have led to "traffic jams", more French troops at the decisive moments very well could have led to a general rout in the allied main army.
The battle also, especially at day one, had many incidents, where butterfly intervention could have accelerated drastically different outcomes. Napoleon winning day one at Leipzig, would still leave him with Bernadotte and Benningsen, but the first would most likely turn around and go for Sweden ASASP, and even if joining up with Benningsen they would in numbers and quality be greatly inferior to the Grande Armee.
If being decisively defeated in Saxony, I very much doubt the allied continental powers could stay in the war - before the battle they were almost as exhausted as the French. With the job done in Saxony it will only be a matter of short time before the British will have to evacuate the continent.
They might opt for a prolonged war, but with the chances of rasing new powerful allies on the continent being remote and more expensive than ever, I guess some kind of peace will be found leaving Europe to Napoleon.
But perhaps the British will now turn their attention to the Americas? It would provide sweet and comforting revenge over traitors and renegades - even some that Napoleon can't do much right now to help...
Waterloo is too late, too many and too good allied troops converging on France.
Regards
Steffen Redbeard