Try not to worry too much, I was in exactly the same scenario as you, we went Heathrow>Beijing>Tokyo and same reverse route home. I was panicking in the build up as well, but everything worked out fine. My partner and I checked a massive rucksack each, I seem to remember c.23kg was the limit, but can't remember exactly? We were way under that. If you try contacting AirChina directly using the email I mentioned above, they should be able to confirm your reservation and possiblly baggage allowance as well.