You may be able to check in your bags when the Air NZ counters open to check in the Los Angeles flight around 12.30 or 1pm, although I've been told by other airlines at LHR that there is a restriction on accepting bags too early for flights.. but that may be a BA or BD specific restriction.

Hopefully the Air Canada agents at Toronto will be able to tag your bags through to Hong Kong, which would save collecting them at Heathrow and let you just enjoy the morning without hanging out in a grotty airport for hours. Is AC one of those airlines that will not tag bags through on seperate tickets?

Another back-up alternative is to leave your bags with left luggage (around £8 per bag, in the arrivals area at T3 or T1) and collect them when you return to check in.

Enjoy your day in London, and good luck!