I am reading a biography of Benjamin Franklin by Edmund Morgan, and he speaks of troop numbers occasionally. 23,000 here, 12,000 more there, 27 warships under Admiral Howe in 1776 alone.
With England shipping troops and ships and presumably most of their weaponry west to fight the colonists, what measures did they take to secure the islands of Great Britain against a hypothetical French or otherwise European invasion?
By the end of the war they were badly beaten in America. In the waning stages, would this not have been an ideal time for France, already assisting the American colonists, to send troops to England?
It is very noteable that after England lost Calais under Mary I, they very rarely, in all their many wars with France, sent significant armies into France. Similarly, while French soldiers would occasionally raid the English coast, they rarely sent significant forces in. Why? Because it would be extremely difficult to attack the other country's populated heartlands, where huge numbers of militia could be raised, and they'd have no supply lines. Thus, wars were generally fought in other countries; America, the West Indies, Belgium, Spain, etc.
In the Revolutionary War, Britain did find it extremely difficult to raise forces; the war wasn't particularly popular, and previously the British army had been relatively small. They had to withdraw forces from Ireland, and hire German mercenaries. So, there were plenty of able-bodied men who could fight, they just didn't want to. If France had invaded, that situation would have changed.