Simply book through TrenItalia's UK official agency:
http://www.italiarail.co.uk/tickets/IR/IRUK.as p?AFF=ITR
The website is a bit quirky. Enter 'Milano', rather than 'Milan', or the station might not be recognised. Conversely, enter 'Venice', rather than 'Venezia', to avoid having to decide which one of several stations you actually require.
It might be simpler to phone them on 08700 84 14 14.
There are no return fares,
per se. You have to buy 2 singles. For the sample dates I've tried, single fares are as follows:
Eurostar services: �20 (reserved, about 2 hours 20 minutes)
Inter-City services: �18 (reserved, just over 3 hours)
Regional services: �16 (unreserved, about 3� hours).
So you should be able to do the return journey for as little as �32, but an additional �8 will save you over an hour in either direction.
Chris