This is dependent on a PA's experience. If registrars with 8 years' experience (five years of medical school, two foundation years and an ST year) start on 30-minute appointments, then it follows that new graduate PAs should be given similarly reasonable times. They may also need time to get signatures for medications or imaging. Appointment times should decrease every few months in the beginning - with negotiation and based on a PA's comfort and experience.
Over time, PAs should have 10-minute appointments, but how the individual surgery deals with signing medication/imaging will affect times. If a PA is required to discuss medication, but only has a 10-minute appointment, then in essence they are being asked to see patients faster than a doctor, as a PA has to wait to speak about the medication with their supervising GP within the 10-minute appointment time.
PAs should be able to discuss patients with any GP. If a surgery runs a duty doctor system, that person should be the supervisor for the session.