did the touristy thang with the folks a few years back...similar to looking@hk's experience, we did venice, florence and rome from the uk.
from stansted to treviso one-way via ryanair was like 1 sterling (no joke...book in advance), then bus into venice...train to florence, train to rome, out via ciampino back to stansted (i think it was 50 pounds).
the trains and subways arent bad at all....dont expect mtr efficiency nor cleaniness.
ps - venice is really nice but dont spend more than 3 days there....or else you will be wandering murano just to kill time. it felt like a pure tourist town with no locals.