Well, y and w are also called semi-vowels sometimes. It's true that in Welsh w is used for /u/ and y is used for /i/. In the IPA, /y/ is the German ü sound, that is a high front rounded vowel. /i/ is a high front unrounded vowel. In other words, you can pronounce /y/ the same as you would the /i/ just cause your lips to round as if pronouncing /u/. Same with /e/ and /œ/: mid front unrounded and mid front rounded respectively, with /œ as German ö.

A w is after all a double-u. And in Spanish, they call a y, the i-griega, or Greek i. Hope this helps. Thatnks for the list, if you're ever in a library take a look at a Welsh dictionary and marvel at all the double-yews as vowels.