The French aren't dismantling the British empire with a war of scorched earth.
Of course they would not. The question was about the
realistic goals which France and Spain could achieve.
The Gov't has to fear losing power and be occupied before it agrees to that. The scorched earth campaign might be good enough to have Britain give up the colonies, Gibraltar, and a minor bone for the French (a port in India, or maybe Canada, although reconquest of Canada wasn't really on the list of things to do for France).
French already had ports in India: Mehe, Karikal, Pondichery, Yanoan and Chandermagor.
So it would be realistic for them to reclaim their former territories on Coromandel Coast lost in 1754.
Canada seems to me less realistic (and probably less profitable) but perhaps some fishing rights.
For Spain getting Gibraltar back would be a big bonus, at least prestige wise. How important was it for the Brits at that time? OTOH, Menorca is quite realistic: in OTL on 5 January 1782 the Spanish regained control of the island (the Brits got back again in 1798 and lost forever in 1802).
That's still quite a bit, but Britain might go for it to end the war. Anything more, and Britain is likely to fight on, and time is not on France's side.
Exactly. This is why I keep talking about the "realistic" goals: those which could be relatively easily agreed upon by both sides.
France must shut off the naval ports to deny the British Navy a home. I've no idea if that's realistic.
Only in the scenario where the French are landing something comparable to the force planned by Napoleon: something in the range of at least 200K. Even if such a force is not adequate for occupying
ALL British ports it would be intimidating enough for forcing the British government to sue for peace. But this also raises at least couple big questions:
1. Is French-Spanish capable of maintaining at least a local naval superiority for a prolonged time?
2. What is modus operandi of the landing troops? There are at least 2 seriously different scenarios:
2a. The "conventional European war" of that period: food confiscations (and a little bit of a looting here and there) are OK but in general, as Fritz put it, "the subjects don't have to know that there is an ongoing war". The targets of destruction are almost exclusively the military objects (wharves, arsenals, dockyards, etc.). The advantage is that the civilian population is not unduly alienated while the disadvantage is that the opponent's government is not unduly pressed either (unless you start burning property which belongs to its members

).
2b. Something close to the "total war" in expectation that the opponent would be compelled to start talks by a scope of the destruction. The civilian targets are just as "good" as the military/naval ones. With the invading army of 200K even the places like Birmingham and Coventry (and perhaps even Liverpool) are within the reach (if we are talking strictly about destruction, not occupation) and the damage (including potential damage) to the local economy could be great enough to speed up the talks process. A potential offset is an unhappy population but Southern Britain in not Spain - there are not too many places for guerrilla to hide and, anyway, the invaders are not into the permanent occupation. Well, the obvious problem with that scenario is that (with the exception of some "colonial activities") the European wars of that period usually were not fought this way and it would take a considerable change of the existing perceptions at least on the leadership level.