Just a few suggestions - unfortunately several are dead or, by now, too old.

Voice of DEATH - Christopher Lee (or Valentine Dyall)
Vimes - Clint Eastwood (in Dirty Harry mode) has been suggested eleswhere
Carrott - Dolf lungren has the build but not the charisma
Nobby Nobbs - Russell Hunter (the bloke who played 'Lonely' in the series 'Callan - for those old enough to remember)
The ArchChancellor of UU - Brian Blessed (Comes complete with own beard and bellow)
Bursar - Alan Bennett
Dibler - I think Sid James could carry it off
Colon - Bill Fraser
Magrat - Jan Horrocks ('Bubble' in Ab Fab)
Rincewind - Rowan Atkinson
Nanny Ogg - Joan Sims
Voice of Gaspode - Jon Culshaw
"...Been on IMdB - David Jason is good as Albert, too..."
I've always seen Albert as Wilfred Bramble (Albert Steptoe)

Walrus