There are a multitude of reasons and not many of them include the surgeon simply deciding to waltz in late.
Most are organisational, many are due to clinical emergencies. I don't work in an outpatient based specialty but I can clearly give you examples where patients have been kept waiting with good reason.
Eg. One of the surgeons I regularly work with has a morning clinic in which the managers insist a 12:30pm patient is booked. She then has an operating list (with around 7 patients on it) which is meant to start at 1:30pm. It doesn't take a genius that she can't do a 20minute clinic appointment, eat lunch and see & consent 7 pre-op patients (specialty which is junior doctor light so nobody to delegate to) within the time allowed.
Clinical Emergencies I was recently involved in a very significantly unwell patient - we had cardiac surgeons hauled out of clinic, a cardiologist, numerous radiologists and 3 consultant anaesthetists. Other patients did with elective procedures did get cancelled but if it was your loved one who was so very sick you would want them to get the treatment they needed. We cannot separate the elective and emergency part of the workload in NHS hospital.
The private sector largely do as they rarely admit people in an unplanned manner eg. if you rupture your abdominal aneurysm (85% of people die) you don't go to the private hospital to bugger up the vascular surgeons list you come to the NHS and it causes merry mayhem. It is also not financially viable to have large teams of people sitting around 'just in case' although I will concede that some hospitals need to review their emergency theatre slot provision to make room for urgent non emergency cases who end up in the dreaded fast- cancel cycle.