OP, it's not an unreasonable question to ask, but it's not the individual consultant's fault, that's how all NHS consultant contracts are.
This was devised so that consultants would actually stay in the NHS and offer the benefit of their skills and experience, and not disappear into private practice the moment they finished their training, leaving the NHS with just a handful of professors and mostly juniors to treat all the patients. In countries where they don't have an NHS system, you see one of two things 1) all healthcare is state hospitals is private to some degree, and even their state patients have to pay, so in effect the consultants are doing private practice in the state health system. 2) those countries with systems that are free are in effect run by juniors - many state of the art treatments are not offered at all, and the inexperience shows in the sort of procedure they are willing to do - eg they will not agree to do a difficult operation at all, and drugs used are simple ones that are often not ideal for the patient's condition.
The fact is that if a consultant could only work in the NHS, the NHS salary bill would either be unsustainably high (we'd be paying National Insurance of 40% of our salary probably), or the consultants would leave the NHS. The fact is that Britain is unique in the world for having a system like NHS (if you don't count communist countries - and you don't want to see the state of the healthcare there. State of the art ITU or neonatal care, or expensive chemo drugs? Forget it) NHS care is unique in being free, esp for lifesaving treatments - from organ transplants to bypass surgery to Caesarean section plus neonatal ITU (SCBU): treatments which would cost in the realm of £20,000 to £100,000 in many other developed countries, depending on how long the hospital stay was. (Not counting IVF or cosmetic surgery, of course, which is in a different category.) You often hear stories abroad of people whose lives were saved by operations or transplants, only to find that the cost has bankrupted them or their families. Having to sell their homes. Or forfeit their child's university education.
You might also find that if your consultant is a veteran/older consultant, or is very popular (eg with a specialist skill or training that very few people have) she may simply have an extremely long waiting list because too many people have been referred to her. (If a doctor has been working for a long time, it stands to reason that a lot of her patients may be NHS patients from long ago who are still alive and need follow up, who need appointments just like you do). If your condition is supposed to be a straightforward one, there is no reason why you can't ask to switch to a younger consultant or one who doesn't have as long a waiting list - NHS consultants will be diligent in making sure all the records are passed to the new consultant, so you should get care that is just as good. But I would not switch clinicians if your case was complicated.