To my understanding, it hasn't actually been a decline in birthrate. We just had a few bulge years rather than subsequent decline.
edit that...probably is more of a trend in line nationally...
No, you can blame incompetence in the strategic long term planning of primary school places instead of pandering to political demands to expand certain schools or open new ones when birth data told you we had a couple of bulge years then a decline in numbers. This was not unknown data.