There are no direct flights from Exeter to Venice.
The only operator from Bristol is Ryanair. They fly to Venice three times per week, on Monday, Wednesday and Friday. So, for a long weekend, you'd have to fly out at 1845 on Friday (arriving in Venice at 2205). Your return flight would need to be at 2230 on Monday (arriving at 2345). However you need to be aware that Ryanair's flights go to Treviso Airport, not to to Venice Airport.
Current RETURN fares, PER PERSON, including taxes and charges (but not the £5 administration fee, the £5 online check-in fee or checked baggage fees) are as follows:
3rd Sept: £116.53
10th Sept: £46.53
17th Sept: £47.88
24th Sept: £60.13
http://www.ryanair.com/en
The ATVO bus from Treviso into Venice will cost you €9 each for a return ticket.
Inbound timetable:
http://tinyurl.com/3yplesu
Return timetable:
http://tinyurl.com/36mbdgj
Start your hotel search here:
http://www.tripadviso...ce_Veneto-Hotels.html
For a guide to expenses (and much more) see Wikitravel:
http://wikitravel.org/en/Venice
Chris