I sincerely doubt Joseph would be anything but a figurehead, based upon his 'reigns' as King of Naples and then Spain.
IF ( and that's a big if ) Talleyrand is the man behind the throne ( rather than a coterie of Marshalls and/or other politicians ), then he will offer concessions to Uk to get peace. Definitely and of blocus and concessions on trade ( all the french empire open to british goods, with reduced tariff or maybe none for a time ), likely on colonies, possibly on land not annexed to france ( confederation of the rhine comes to mind, as do ionian ( sp? ) islands, Istia and Dalmatia ) and, if absolutely necessary, some slight concessions on lands recently annexed to France in North Italy ( ie, maybe the 1805 borders can be abandonned in favor of the 1802 ones ).
But the last items are only a last contingency and Talleyrand will try to get a lasting peace while giving away as little as possible ( as long as it is a true peace ). He will face to much internal opposition if he gives away too much.
Don't forget that at this point, the british showing on land has not been great and that the french have not entered Spain ( and are not likely to ITTL! ). Without the bleeding ulcer of Spain and with the blocus removed, Russia will not attack France and Austria will not dare to ( Prussia is removed from the great power game at this point ). So Uk will have difficulties refusing the peace.
The problems, will be, IMO, the titles of Joseph and Louis. They were respectively kings of the two Sicilies and of Holland at this point. I see no problem with Joseph renoncing his title in exchange for peace; he just became emperor and did renonce that title when he became king of Spain OTL. Louis is more of a problem. He was quite fond of his subjects ( and they of him ), so much that he actually defied his brother Napoleon over this ( which is why Napoleon later annexed holland OTL ). SO he won't want to give up his title. OTOH, it's going to be very difficult to get UK to accept this, esp as Louis is heir after Joseph sons ( he didn't have any OTL but here.... ). I think it may go, with enougth concessions on other point and no real hope in the war against France.