Material from The French are Coming by Peter Lloyd and The Terror Before Trafalgar by Tom Pocock contain evidence that not only were the royalists and assassination attempts financed by Great Britain, but high-level people in the British government were involved, whether or not they were presently in the government when their participation in the conspiracies too place. This includes William Pitt, Lord Whitworth, William Wickham, and William Windham, not to mention involvement by the Royal Navy and various British subjects employed in the intelligence business.
And information from these two volumes backs up the related material in Vincent Cronin's biography of Napoleon as well as Horrick's book.
From The French Are Coming: The Invasion Scare 1803-1805 by Peter Lloyd, 44:
‘One August night in 1803, a squat, barrel-chested Breton was secretly put shore from the Royal Navy brig ‘Vincejo' at the foot of the cliffs near Biville, between Dieppe and Treport. His name was Cadoudal, but he was more commonly known as Georges. The name already possessed a fearful significance for Napoleon. Georges was the agent of the Comte d'Artois, the younger brother of the executed Louis XVI and presently ensconced at Holyrood House on a British pension. His mission was financed by British secret service funds and its objective was to draw together royalists and Jacobins in a conspiracy to ‘kidnap' the First Consul and restore the Monarchy. However, there is little doubt that it was, in fact, an assassination plot; Georges was a ruthless and dedicated killer who had come within an ace of blowing up Napoleon's coach in a Paris street on Christmas Eve, 1800. In the view of monarchist Europe, Napoleon was a parvenu: his regime had no legitimacy, and the code of behavior between rulers simply did not apply in dealing with him; even so, complicity in such thuggery shows how the invasion threat had lent a paranoid intensity to British fear and loathing of the Corsican Ogre.'
From The Terror Before Trafalgar: Nelson, Napoleon, and the Secret War by Tom Pocock:
Lord Whitworth became the first British ambassador to France after the Peace of Amiens was signed. ‘But Whitworth, also secretly, had charge of a key element in the British intelligence network established by William Wickham, who was ostensibly head of the Aliens Office in Whitehall. Both Whitworth's first secretary and his private secretary were experienced intelligence officers, skilled and discreet in watching, listening, and enquiring.': 56-57.
‘Throughout the months preceding the attentat the British government-Windham, and even Pitt-had been involved in financing French dissidents and helping transport them to and fro across the Channel.': 68.
‘Georges [Cadoudal]…predicts that Bonaparte will be cut off before two months, though he professes not specifically to know of such intention, seems to thing such a course of proceeding legitimate and has thrown out the idea to Pitt as he has before to me. Not necessary to say that no countenance was given to it.': 68; an extract from Windham's diary.
‘A month later he recorded a conversation with Pichegru, who ‘talked of a design to cut off Bonaparte by assassination and of the general instability of the government, to which latter opinion I felt inclined to assent. On the other hand, having before expressed my opinion, I did not now say anything.' A few days later a third dissident, Chevalier de Bruslart, ‘made wild proposals of carrying off, or cutting off, Bonaparte, to which I pointedly declared that a British minister could give no countenance.' Soon afterwards both Frenchmen returned to France taking with them large sums in British gold, leaving Windham and others well aware of their intentions.': 68.
‘There were soon to be more mysterious arrivals in France. On the night of 23 August 1`803 a darkened sloop glided towards the cliffs of Normandy ten miles east of Dieppe. She was commanded by Captain Wright and engaged on ‘a secret and delicate service.' On a rising tide she closed the shore and a light gleamed from the cliffs 300 feet above the sea. A boat was lowered, seamen pulled on muffled oars and passengers scrambled ashore on sands below an unobtrusive fold in the cliffs. The place, half a mile from the farming village of Biville, was well known to smugglers for it was a deep cleft running inland, rising steeply to where they had cut a path through the sandy soil to the fields above, from which it was as obscured as from the sea. A rope had been lowered down the path, where it rose most steeply, to help one of the arrivals, a corpulent, powerfully built man, and at the top a horse was held ready for him so that he could ride, while the others walked with their guides, to a safe house, the remote farm of La Poterie. This was Georges Cadoudal, the conspirator of the attentat in the Rue Nicaise, on his way back to Paris.': 106-107.
‘Another resident of Walmer Castle privy to much clandestine activity was a tall, thin, ungainly man of forty-two with a pointed nose and a receding chin and the complexion of a drinker. This was William Pitt, the former Prime Minister and promoter of secret operations, still privy to the twin approaches in the prosecution of the war: the broadsword of national defense and the stiletto of espionage, subversion and perhaps assassination…Pitt, as Prime Minister, had been aware of the plans to assassinate Bonaparte in 1800, which had so nearly succeeded, but had been careful to distance himself from direct involvement. Now Georges Cadoudal was, he knew, to attempt an even more ambitious coup: the assassination or abduction of the First Consul followed by the restoration of the Bourbon monarchy.: 110-111.
‘[Cadoudal], a heavily built, red-haired Breton aged thirty-two, had been a counter-revolutionary since 1793, commanding Chouan insurgents as a royalist general in north-western France; after escaping to England, he had organized a training camp for guerillas in Hampshire and planned the abortive attempt on the First Consul's life in December 1800.': 130