How would a British Londoner, pronounce the words "Thunder Muscle"?

I watched a show called “The Increasingly Poor Decisions of Todd Margaret”. The premise is an American needs to sell an energy drink called “Thunder Muscle” in England.

I have tried to listen carefully and it seems the locals all pronounce the words as “Funder Musoo” The “Th” sounds like “F” and the “scle” sounds like “oo”. Is this a realistic regional pronunciation? What is the region?

Fanks, Mate.