In "Winnie the Pooh", Why isn't the Hundred Acre Wood plural?

Winnie the Pooh was written by an English author, and thus follows the British English usage that wood is both plural and singular.

Apparently woods is an American construction. In fact, I can't verify that woods is a really a word, but it certainly sounds like one to my American ear.

Wood and woods are interchangeable.

wood |woŏd|
noun
2 (also woods) an area of land, smaller than a forest, that is covered with growing trees : a thick hedge divided the wood from the field | a long walk in the woods.

_{from NOAD}

EDIT: According to @HaL, wood is standard in BrE and would have thus been used by A. A. Milne, an English author.

(In AmE, we do indeed often use woods)

This is hard to explain, but I would say, as a British speaker, that 'woods' has the flavour of a mass noun, whereas 'wood' is a count noun. When you're talking about a specific small forest, it's a wood - Hundred Acre Wood, Bricket Wood, St John's Wood, Highgate Wood, South Norwood, Goodwood, etc. But if you're talking about some indeterminate patch of trees, it's woods - you might be lost in the woods, strolling around the woods, not be out of the woods yet, etc. Perhaps it's a wood when seen from the outside, but woods when seen from the inside.

Having said that, where i grew up, we used to play in Wivenhoe Woods. Googling, i do find lots of references to Wivenhoe Wood, but the usage i heard was definitely plural, and that usage also finds attestation on the internet. The nearest big supermarket back then was at Highwoods. Perhaps this is an Essex quirk; it wouldn't be the only one.