It was often the case that Canadian forces during WWII were included in formations described as "British," because they were under British command.
However, there don't seem to have been any Canadian units in XXX Corps at the time. It's important to understand that the composition of British corps, divisions and brigades was not fixed. Units could be and were re-assigned as the higher commanders saw fit. This was quite different from American practice, where divisions had a fairly fixed composition.
For example, the Guards Armoured Division had had seven changes of the higher formation it served under between D-Day and Market Garden. Starting off in VIII Corps, it had changed to XII Corps, Second Army, back to XII Corps, then VII Corps, II Canadian Corps, VII Corps again, and then XXX Corps, where it served from 28th August to 12th December.
Addendum: I've just learned that there were Canadian units serving in XXX Corps, albeit small ones. The 20th and 23rd Field Companies, Royal Canadian Engineers, were attached to the British 43rd Division's Royal Engineers, and did much of the evacuation of survivors of the Battle of Arnhem southwards over the Neder Rijn on the night of 25/26 September 1944. This was Operation Berlin, and it was far better organised than the rest of the battle.