As your link demonstrates, mush, there is little chance that the law, as it stands, will support a successful prosecution of a practice which, without any doubt, is widespread. The criteria to be met when two doctors authorise an abortion are so wide (or might even be described as so vague) that it can quite easily be argued that the health of any woman who asks for abortion – even if she specifically says it is for gender selection purposes - will be at risk if her wishes are not met. So, to answer your two questions:
“should it be illegal to abort purely on grounds of gender?”
It certainly should. If everybody selected only male children (as is the choice where selection is practiced) the population would very quickly become unbalanced, with obvious undesirable consequences.
“or should it be no business of the authorities to interfere in cultural gender selection?”
As above, I believe government has a duty to ensure that the population does not become significantly unbalanced by artificial means because the consequences would be somewhat unpleasant.
And so to my usual drum banging. This question has only arisen in the recent past following the influx of vast numbers of people whose culture is considerably different to that widely held in the UK. I cannot recall thirty or forty years ago there being any widespread demand among the population already here for gender selective abortions. Indeed it would not have arisen because the vast majority of people place equal value on both male and female children. The same cannot be said of some of the imported cultures. But, instead of being told from the outset that gender selection was not part of the culture of the UK and that equal value should be placed on children (and indeed adults) of both genders, newcomers were encouraged to believe that they could continue with their cultures and beliefs. We’ve imported alien cultures to the UK and not discouraged some of the less pleasant practices that go with them. So we should not be surprised that problems such as this arise.