Yes - you remain in the same terminal (Terminal 3), but you will need to clear Canadian immigration and make your way over to the international departures area (where you will go through security).

Once thru security, you will have access to the BA Galleries Lounges (Club on the left, First to the right. The F side features table-service pre-flight dining, while (AFAIK) Galleries Club has a buffet. Unlike the old YYZ lounge, the Galleries Lounges are located very close to the BA gates (I used Gate 33 my last time through YYZ).


Map of T3:

http://gtaa.com/local/files/en/Maps_T3.pdf

(The BA Galleries Lounges are upstairs. next to the KLM lounge that is marked on the map.