Hello Pat
I do not know if you can book your luggage all the way through from Manchester, somehow I think it might be difficult, considering you will be using 2 different airlines.
At the moment, although things can obviously change, Air China use Terminal 3. Terminals 1,2 and 3 are fairly close together, so if you land at one of these from Manchester you should be OK. If you land anywhere else the terminals are a long way away and you need to use buses. Terminal 5 is 2 miles away by road and taxis are extortionate. Expect to pay at least 20 pounds. There are also licensed porters to help with luggage at a cost of around 10 pounds
I know that the flight timings are an evening departure, but when I go next year,I will have a night at Heathrow before departure.
I understand also that Air China are fairly tight with quite severe restrictions on hand luggage.
I know you will have an enjoyable holiday,
Peter