Do I need a qualification to fly a drone commercially?

No. There is no qualification requirement for commercial drone work in the UK.

What you do need is the right operator ID and flyer ID for the aircraft you are flying, and commercial drone insurance.

What a qualification gives you is reach, and the ability to fly in places you might not otherwise be able to. The A2 CofC lets you fly closer to people than the basic rules allow, and the GVC opens up operations which simply aren't possible in the open category.

No qualification or certification grants permission to trade, and anyone implying otherwise is selling you something they shouldn't be!