If I were the OP and hadn't sampled the T5 lounge then I would fly back to LHR the day before making sure you stay under the 24 hour rule.
Enjoy the delights of the Sofitel then enjoy the lounge before your flight to YVR. You'll have had enough time in Venice to see it so rather than have the bother of connecting the same day and maybe rushing to make your YVR flight.......take the less stressful option.