Yes you can.
The cruelty line (and YES, it is cruelty) is 0300 1234 999.
The Animal Welfare Act 2006 introduced things called the Five Freedoms (in simple terms.)
They are (this is from memory BTW so might not be the correct wording but they are right):
Freedom from hunger and thirst (And that also means providing the right type and a amount of food to keep them fit.)
Freedom from discomfort (considering that the cat is obviously uncomfortable this one applies.)
Freedom from pain, injury and disease (considering the amount of diseases etc that can be caused by obesity, this applies)
Freedom to display natural behaviour (Well, if an animal is obese then they obviously can't do a lot of their natural behaviour )
Freedom from fear and distress.
If the cat has a health problem that means it's obese (such as hypothyroidism ) then they need to make sure that they take him/her to the vets and get on treatment (which is another thing that pet owners have to do under the Act - proper health care! )