I just lost a huge post I had here... so here's the tl'dr version.
Is there interest? You have Emiko's con, Con-G and another con in Guelph in November that I forget the name of, so there's already a decent crop of local cons (oddly enough i'm not aware of a convention in London, which is a bigger city...). Although you have a different slant on things, people will still only attend one. Is it possible to add onto another con and have a costume focused track?
Where to put it? October seems to be your best and pretty much only bet if you don't want to end up in a cruddy month or competing spot. Remember that your core audience will be there September - April.
Plus costume con 32 is being held in Toronto in April of 2014
Actual locations can be tricky here... as i'm not sure if Emiko's mini con is still heavily connected to the Universities there. They moved out to a hotel this year, but before they were on campus. University locations are great for mini cons as you can get a bunch of rooms for workshops and panels. I would get a hold of someone from that convention and test the waters about having a cosplay oriented convention.
Learning? Since you seem to learn hands on I would suggest volunteering at Anime North, Emiko's mini con and Con-G to get a taste of how cons function. Figure out what works and what doesn't work.
You'll also need a dedicated group of people who want to make this happen. Many conventions fail because of a lack of volunteers and poor communication.