Can any agency take action for extremely narrow seats travelling to Toronto by Air Transat & Thomas Cook plane?

London-Toronto and return flights from London to Toronto by Air Transat and Thomas Cook is 7-8 hours (a long haul flight). But the seat pitch is so narrow that only slim people can sit easily. All bulky people have to be very much uncomfortable. Is there any agency or authority who oversee these two airlines are breaking rules.

Try spending a bit more money and going by Air Canada instead.
If you go for the cheapest, this is what you're going to get.

no lose weight

No.

And narrowness of seats has nothing to do with pitch. That's the spacing from one seatback to the next behind it.

Air Transat is a budget carrier and their seating arrangement reflects this. The more seats that they can cram in, the lower the unit price is for everyone.
If you want wide seats with plenty of leg room, book first class with BA or Air Canada and pay 5 times more for the privilege.
I doubt if Air Transat is breaking any aviation rules.