I already own a mixed use property – a shop with a residential above. Am I considered as first time buyer if I purchase a residential property now?

This will depend on whether the residential part above meets the definition of a dwelling. A dwelling is a self-contained unit which consists a living area, bedroom, kitchen, bathroom and toilet and has its own independent lockable access. If it meets the definition of a dwelling it means you already owned an interest in a residential property. You are not considered as a first time buyer.