Cardinal Wiseman, first archbishop of Westminster 1850-1865,
Cardinal Merry del Val, Secretary of State to Pope Pius X,
Cardinal Bourne, archbishop of Westminster 1903-1935,
Cardinal Hinsley, archbishop of Westminster 1935-1943,
Cardinal Godfrey, archbishop of Westminster 1956-1963
Cardinal Heenan, archbishop of Westminster 1963-1975
and, more exotically,
I think all of these would have been products of the junior rather than the senior seminary.
Can`t think of any other famous alumni at the minute.
UPDATE: Thanks for the additions in the com box:
Archbishop Charles Petre Eyre, first post-Reformation Archbishop of Glasgow.
Joe Tasker, the mountaineer who died on Everest in May 1982 (I`m sure I`ve met his parents)
Patrick Joseph McAloon, Prefab Sprout, 1968-1975 . (Do most Catholic institutions produce pop-stars?)
To these I think we should add bishop Hugh Lindsay, bishop emeritus of Hexham and Newcastle who is known to most Catholics in this country for his letters to the Catholic press.