OP, you got good directions above, but the signage in YYZ's Terminal 1 was changed a couple of weeks ago.

Instead of looking for a sign that reads "International Connections", look for a sign directing you to the "E" gates. As mentioned above, you go through a quick document check only--no need to claim checked baggage or to reclear security. On the PDX-YYZ flight you'll be given a Canadian customs/immigration form, and you'll need it to pass the immigration counter that leads to the "E" (international departures) gates. Depending on where your flight from PDX parks, you'll probably complete the process and be at the departure gate for the LHR flight within 15 minutes or so of arriving YYZ.