As someone who deals with both aviation and GMC cases I will add a couple of comment as follows:
Level of fines - undoubtedly the magistrates would have had regard to the defendant's means and would have pitched the level of the fine accordingly. Higher the income generally means the higher the fine.
GMC - the GMC would generally be concerned about matters that affect a Doctor's ability to practice and the confidence of the public in the medical profession. Therefore what might be seen as paperwork errors (overlooking an expired medical etc) are things that could directly translate into a Doctor's professional capabilities and may be more serious in a GMC context than other actions that would be outside the scope of a doctor's duties/responsibilities.