Hi and welcome.

I don't know anything about persians but you acknowledge that the food you are feeding may not be the best.

You might want to change to a wet/dry diet that is meat based without cereal bulking it out.

The cat eats less and there is less likelihood of conversion of carbohydrates to fat. Good wet foods are:

Natures Menu available at Pets At Home
Bozita available zooplus
Tescos Luxury/premium
Hi-Life

Dry: Orijen from zooplus

You could try raw too.