Killer Instinct by Zoe Sharp
Charlie Fox was no good at being the nice girl her parents wanted, so she joined Her Majesty's military and acquired a new set of skills. Now she puts them to use teaching self-defense to battered women in a refuge - passing on the finer points of roundhouse kicks, running like hell, and breaking a cheekbone when necessary. Her skills come in even handier when she takes a job working security at the Adelphi, a hot new club with an enigmatic owner. And they come in handiest of all when a rapist appears to be targeting the desperate women at the refuge, and someone with a very nasty line in knife-work appears to be targeting Charlie's friends.