The vowel sound in English might seem detached from the spelling but it isn't. The pronunciation tends to be consistent for the same word, wool is generally wuhl within the range of the accent but wooly, to have the nature of wool is more extended it kind of falls between uh and oo.
But someone from Liverpool would likely pronounce book and cook as BOOk and COOk, though personally I never did.
@yvanspijk
The phoneme 'ʊ' is that short uh and it's unfortunate that the description for English uses words with 'oo' rather than unambiguous words like 'lug', 'but' and 'multiply'.
English is hard enough to explain as it is. And it remains that Worcester is a weird word. As it happens I think the Etymology is wrong as well.
I think Vertis and ceastre evolved into Wercester. We still have an existing Roman Wall.