canada immigration question

I am in UK now.I submitted my documents on 8 Jan 04, I got the file no. on Feb. it is in processing now. my visa will be expired at the end of Nov. I don't want to extend my visa in UK. What I am afraid is: if I go back China, I might delay the second letter, like interview or medical test, is there anyone has the same problem? anyone has experience for that to share?

I do appreciate for any helps,

thanks a lot