Volume is the single biggest factor, and it has a far greater effect on price than most people expect.
Consider two patients. One wants subtle definition along an already reasonably strong jawline. That might need just 2ml. The other has a naturally softer jaw angle and wants genuine structural sharpening from ear to chin. That can mean 4ml or more.
Same practitioner, same clinic, potentially double the cost, purely down to how much correction the anatomy requires to achieve a visible result.
Practitioner expertise is the second major factor. The jawline runs close to the facial artery and several important nerves, and achieving a straight, natural line rather than an uneven or lumpy one takes real technical skill. This is one area where inexperienced injection is visibly obvious, which is exactly why expertise commands a premium.
Location also matters. Central London, and Harley Street or Mayfair specifically, runs higher than regional UK clinics, reflecting both overheads and a concentration of specialist injectors trained specifically in lower-face contouring.