No visa requirements if you hold Canadian visa.
However, please note that in case of lengthy delay @ heathrow, you may not be allowed land-side for hotel, etc if there are long delays or cancellations. Cancellations are rare so you should not worry much. 5 hours is enough time to change terminals.
Once you land at heathrow, follow "flight connections" signs.......don't waste time, just keep following "flight connections" signs.
This can take some time, so make sure you change terminals as soon as possible.
On heathrow website: http://www.heathrowairport.com/heathrow-airport-guide/flight-connections/terminal-1-international-connecting-to-terminal-3-international
Most shops @ Heathrow airport accept major currencies......so there is no need to obtain pounds, you can make payments in dollars.
Arrive early at the gate, for the flight to Canada as there is passport/visa check before boarding flight to Canada.