You *may* apply before receiving your police certificates and enclose a letter showing that you have applied for them and are still waiting to receive them. Others have done so and been successful.
Would I recommend it? No. I say put a complete package together as specified by CIC and then wash your hands of it. Having things outstanding which you must wait for the visa office to receive your application, begin to review it, then request these documents from you, pray to God they match it up when you send it, then wait for them to realise you have sent it and start processing your file again etc etc etc can cause unnecessary delays on the back end of the process.
You will be dealing with a quick office, which is also highly efficient, and in such a case I suggest ensuring you send a complete application to London, thereby making it easier for them to do what they need to do, which they already do so well.
