it can be practice that the antenatal clinic will only do the admin of the 18 week scan. To get a dating scan or the 12 week NT scan is still 'optional' and many ppl find they organise it through their GP (this is more often the case if you are going public...i remember when i went private with DD1 it was organised through my OB, but this time i had to do it all through GP itms).

HTH!