Best Answer - Chosen by VotingI'll suppose you're using this layout:
http://en.wikipedia.org/wiki/File:KB_United_Kingdom.svg
and that UK and US International are about the same.
Microsoft says:
' + c = ç
' + a, e, i, o, u, y = á, é, í, ó, ú, ý
" + a, e, i, o, u = ä, ë, ï, ö, ü
` + a, e, i, o, u = à, è, ì, ò, ù
~ + o, n = õ, ñ
^ + a, e, i, o, u = â, ê, î, ô, û
I don't know about the other combinations... And if this really work, but give it try. =]