I am examining a MS recipe book from Devon (England) that probably
dates from around 1750.

There is a recipe for Floating Island, thus:

"To make the floating Island.
Take the whites of two eggs with two spoonfulls of red Currant jelly,
beat them together for two hours or till they are very thick; then put
it with a Knife and a Spoon into a deepish China Dish and rise it as
high as you can into the shape of a sugar loaf. just befor you serve
it in, put into the Dish half a pint of raw cream with a little wine
and sugar: then move up the Island and it will float about."

I suspect that this recipe considerably predates the earliest citation
in the Oxford English Dictionary, which is 1771 and from the USA.

Can anybody throw any light on the origin of this dish, and its early
variants?

While on the subject of old English cooking, the Holiday Inn in
Russell Square, London, has on the menu for dessert "Sticky Richard."

On ordering it, I found that it is the old English standby dessert
called "Spotted Dick", consisting of a sponge pudding with sultanas -
served with custard sauce.

Is this a common bowdlerization of the two original words?