This is from the Devon & Cornwall police website.
"What is Positive Action?
Positive Action is a range of measures and initiatives that we can lawfully take to actively encourage people from communities that are under-represented to bring their talent, experiences and expertise to our organisation when applying to join us.
When applying to join us, our selection processes are no different. Through the use of Positive Action, we do not seek to remove competition, rather, to allow everyone the same level of opportunity where final selection for a post is always made on the merit of the applicant."
If you think it's unlawful, it might help if you identified the legislation it contravenes.