Hi Mr.Singh,
Don't worry at all. I assure you there will not be any problem for the country from where the visa is stamped. They just look only whether you have a Canadian visa on your passport. No one is going to question you how you managed yo get the visa stamped at London without traveling there. The rule India does not allow to send the passport by mail (means) post to out side country. You are always allowed to send the passport out of the country with an authorized person by yourself. So it can also be sent by your relative or friend traveling to and back to UK.. Me and my wife traveled several times from different airports (2009 March-Mumbai, 2009 May-Chennai, 2010 and 2011 - New Delhi, ) to Canada with our visa stamped at Abu Dhabi and we were not in Abu Dhabi. We sent our passports by courier and got back by courier while we were in Saudi Arabia in 2008.
Travel with peace of mind. no need for tension. Wish you all the best for a happy landing/