There are of course plenty of examples in the eighteenth century where the situation was reversed between France & Britain. French support for Scottish and Irish rebels or for the Americans in 1775-1783. In all the examples above, money, guns, and other supplies were given covertly without anything being stamped "from France for the purposes of hurting England," yet that was the purpose. If we were to apply the same logic used to defend the British government's support of emigres to France and the above examples, one should be able to argue that French guns and money arriving to support anti-British rebels got there "accidentally" and had nothing to do with undermining the British Empire in the great power game.
Those expecting some kind of 19th century version of an Operation Mongoose where there are letters from high-ranking British officials saying "I want you to kill Napoleon and here's 200 pounds for that purpose," won't find anything so direct. First, before formal, bureaucratic intelligence agencies one didn't write such things down. Second, there's been 200+ years for the individuals and governments involved to destroy any incriminating records (something public figures of the period regularly did to "clean up" their image for posterity). That's why Roberts has found only tangential evidence on the subject. But I find enough circumstantial evidence that the British government knew they were supporting assassination attempts by giving money to Cadoual et al., with a wink and a nod.