I just found this while doing some research on the Canadians at Aldershot - Why the Army Group commanded by Montgomery was called 21 Army Group:

"The arrival of the 2nd Division brought Canadian troops in Britain up to 56,000 and on Christmas Day 1940 the Canadian Corps came into being, commanded by Lt Gen. McNaughton, who had initially brought over 1st Division. In the next eighteen months three more divisions and two tank brigades arrived, a second Corps was formed and all were put under command of a newly created First Canadian Army. This was the Army which fought alongside the British 2nd Army throughout the subsequent NW Europe Campaign, the two numbers being combined in the title of 21 Army Group commanded by Field Marshal Montgomery."