They are fully funded but expensive to do and some GP's either refuse to do them, or only do them under some justification. A while ago we tried to get my partner's level checked, but because he was already supplementing his GP refused saying that, as he was already supplementing he wouldn't be deficient, and so there was no need to test.
It would not be unreasonable to have a full work up of tests which could include Vit D3 once a year as a matter of course in the interests of prevention/maintenance.
Because of the mounting evidence showing that many people are defficient I would be concerned with keeping a GP who catagorically would refuse such a test; I would not feel that they have my best interests at heart.
Interesting topic, Although it is nice to know exactly where you sit I think the reasoning used by your GP may well be justified in that if you are taking large supplemental doses the only reason to have it tested is to make sure you weren't "too high" and that is not clearly defined anyway. From a medical ethics view it fits into Justice and basic medical decision making. Do not test markers of a heart attack if one has come back, do not do daily blood tests if they will not change your management etc, it is of course something that can be discussed with them. I feel sometimes though that people forget these doctors have done 6 years of training plus an additional 5-30 years of practice? Doing tests may make one feel better, but not change anything....