How many hours do I need to work to be considered a part-time worker?

The Part time Workers (Prevention of Less Favourable Treatment) Regulations state that a part time worker is one who is "not identifiable as a full time worker".

A full time worker is a worker who is regarded, through custom and practice, in that establishment, as full time.

In other words there is no specific formula. In some companies, workers who work 30 hours and above, might be customarily regarded as full time, whereas in another company it might be only workers who work more than, say 35 hours a week.

To be considered as a part time worker in your company you have to be identified as someone who is not customarily regarded as full time.