I am, I would agree here with Johnnyreb, by 1805 the Jacobite are not just a bunch of dreamers who lost all support from the Catholic scotts, even of those of Highland clans, Lowlanders as Protestants themseves wil be reluctunt to interfere with all this crap.
What if Napoleon had managed to land his forces at Dover or in Cornwall - the atter is less pausible for the British would use nerrowness to the defile for their advantage. Here the examples of Waterloo are most appropriate.
The British had several Highlands regiments of Scottish fusiliers in 17 (? AFAIR) squares on the top of the small plateau. These brave men fought pretty well but that was not enough to keep them from being smashed by the frontal attack of weakened badly and having lost their trot by that unfortunate cuirassiers under command of Michel Ney. The French attack, desperate as it was in the final cut of the battle made the Alied line to start tearing apart but the French no more had force to advance. The Engish skirmishing lines were formidable shooters but they were weak against the fast cavalry attack - Napoleon knew that much - that's probably he ordered to attack the Duke Wellington's centre with massive cavalry. The Highlanders squares screening redcoats skirmishers were smashed but they fulfilled their task - the French cavalry was worn out. Relief played its bad joke on the French. An the rain during the night before the battle, too, contributed to the final defeat of Napoleon. But for the rain, he had not to wait until 10 a.m. to start the attack. Shoud the French started the battle only 2 hours earlier, Blucher would have arrived on the ashes of Hougoumont and seen the frightened refugees in red coats swarming the roads in attempt to survive.
And then already it would be Cambronne's turn to cry out: "Brave Engishmen, you've fought well, now surrender with honour!", and one of british officers wouf respond to that: "Sh...t!" and died bravely and foolishly in the salvoes of the New Guard.
1) There's the fact that Napoleon knew British tactics and tricks
2) His troops back in 1805 were much more experienced in fighting than the British without war in Portuga and Spain
3) His troops had extremely high morale and were probably the toughest fighters in Europe in massive dogfight (Napoleon was surprised when he saw the expertise of the same level in Russian grenadiers in the battle of Preussisch Eilau) while the British were genarally the better shooters.
4) Grouchy would hardly be entrusted to lead something more than the division because there the whole Pleiad of formidable generals still alive on the year 1805.
So, my guess, with all respect to Engish army, that if RN somehow were unable to intercept the Envasion Fleet, the Grand Armee is expected to be in London in one-two weeks after landing having defeated whatever British forces brought before it.
Say, the way Britain is firm is on seas, that holds the truth for the French' firmness on land at that time.