While I have not lived in London for several years, I did grow up there and do visit family there occasionally. I hope these suggestions help.
If you have access to a car, there are a few places you might want to try. I would guess the largest fine men's store would be Channer's at 581 Wonderland Rd. S. which carries Canali, Boss, Burberry, Lacoste, AE shoes and the like. Their website is http://www.channers.com/homepage.asp
In the north end of London, on Fanshawe Park Rd. is Tyner Shorten Clothiers. They carry a good selection of Samuelsohn and also Pal Zileri suits and some interesting European sportswear including Paul & Shark. The merchandise is a little more subdued than Channer's (no Armani or Diesel jeans). The website is http://www.tynershortenclothiers.com/business.html
It is not a terribly large store, and if you are not seeking conservative clothing, might not be worth a special trip.
Downtown, at Queen's Ave. and Richmond is David E. White Clothiers. They also have Samuelsohn, AE shoes, Paul and Shark sportswear and the like. They would fall between the other two in terms of the fashion offered.
Fodemesi Shoes on Dundas, west of Richmond St. is one of only three Alden dealers in Canada. They concentrate on the othopedic shoes, but do have stock from the "New England" line available. While one usually has to place an order, it might be interesting to see if they have anything in your size. Their website is http://www.fodemesi.com/
As for vintage clothing stores, you might want to try Richmond, south of Dundas, King Street between Wellington and Ridout. Ask around: the area around the new Covent Garden Market and the John Labatt Centre may have some offerings. Also try Novack's on King St. which has outdoor clothes and may still carry some interesting army surplus.
As for a tobaconnist, I believe there might be one on the north side of Dundas, west of Richmond. On your way, do stop in to Kingsmill's, London's oldest department store. It is independent and while there is only a small men's department, it is worthwhile to take a look around. The furniture and china floors are excellent and they do have a lot of British goods available.
Unfortunately, in the late 1980's, Robert Campeau (remember him?) transformed the existing shopping centre downtown into "Galleria London". When it opened, there were branches of many high end stores such as Polo/Ralph Lauren, Harry Rosen, and the like. These caused the smaller established stores in the area (Scott's, Bob Dowler, Doug Powers, Boughner's) to go out of business. The recession of the early 1990's caused the mall to fail for the most part, and now there is really not much left in the downtown - a real shame.
There are two big malls in London: Masonville at the north end of the city and White Oaks, in the south end, not far from Hwy 401. Neither have anything special to offer and really aren't worth a special trip.
You can actually do quite well in London if you like conservative business wear. However, for more interesting shopping, either Toronto or Detroit beckon.
Do let us know what you think of "the Forest City" when you return.