I have looked at the Canadian law about fixed term elections. It inserts an explicit provision in the Canada Elections Act regarding the power of the Governor General to dissolve Parliament.
There is nothing remotely comparable to the Canadian provision in the UK legislation.
I believe that language was inserted because it was so obvious. Under our constitutional monarchy there is no way short of a constitutional amendment for parliament to pass a law that affects the reserve powers of the crown. Of course one thing that is different in the UK is the fact that there is no written constitution in the first place.
What if Theresa May simply used her majority to repeal the Fixed Term Elections act and THEN asked the queen for a dissolution?