I would assume that there would be a secure area such as left luggage to keep your luggage while you venture off for a small charge, unfortantly the airline normally wont accept bags for check in 8 hours in advance. If i is anything like New Zealand their will be a belt after customs where you able to place your bags and it will be kept to be loaded on to your next flight.
Again unfortantly by law you have to clear customs at the first point of entry into a country, in your case being Halifax, this wouldnt be the case on te return as you have already departed from within Canada, and would only need to collect your bags when you arrive at Heathrow.
Hope this helps