nessa O199 ,
I can share my experience with you When I applied my PR, I was in Canada as an international student with study permit. I applieed in Buffalo and left Canada as my study was finished. I cam back home Bangladesh. I gave them my home (Bangladesh) address for all correspondenc. They sent all documents to bangladesh. The problem was when it was time to send the passport. But I was able to send passport to USA by DHL. Buffalo office only post passport within USA and Canada. So I had to use one of my friend's address, and then she post my passport by canada post.
Now about your cas, If would like to go back, I think there will not be any problem. If you can send passport to London by post and they send it back to you, it will be fine.
As you say London takes very less time for processing, so your condition may delay your case only.
Now, the decission is completly yours. If i were you, and I can aford the visa, I would stay in London.. Just to make the process simpler..
By the way if you are from Visa exempt country, you don't have to send passport any more..
Thank you so much !!! for share your experience, It is really helpful to read something like this right now.

I went over my post and I wrote my student visa expires the 23 of sept. ( already expired) Actually is January 23d 2012, sorry! I changed it already.
So... yes.. It is quite tricky cause we would have to do the same that you did with yours, sending it through a friend,
the thing is that... we really want to leave...
we thought even to apply for a tourist visa to visit his family in Canada but its so likely that would be rejected. I think.
So.. I think we would have to stay in the end.. a little bit more....