In answer to your question: I suspect more than we know. Food allergies manifest themselves in many ways, from excessive grooming, to diarrhea, to excessive itching. Sometimes food allergies can be misdiagnosed as a behavioral issue.
You mention "biscuits": is this dry food? (In the USA, we say 'kibble'.)
A quick google of the ingredients in RC Hypoallergenic indicates that the first ingredient is pea, followed by a duck byproduct. Some cats are allergic to peas, some to fish, some to chicken; allergies can be severe to just a little discomfort.
What symptoms is/was your cat showing? And how is your cat doing on the RC hypoallergenic food.