Yes it is a slow start to spring, even in a greenhouse because of less sun meaning less warmth. However that does not apply to windowsills of a house.
I think it unlikely that light levels are affecting the germination of seed in compost. More to do with temperatures which are also low. (In fact the mean maximum temperatures in Britain have shown a small decrease in the last 17 years, possibly due to jet contrails – the overall warming trend is because nights have been warmer).
As for onions, my Red Baron are coming up fine, at the same speed as other onions. But the Mammoth are slow and that may be becuase of seed being older. I fear this can often be the case if seeds given the correct temperature are slow to germinate, and then do so erratically rather than all coming up together. Last year my main sowing of Sturon produced hardly any onions, so I had to plant some sets.
Parsnip should be up after three weeks, if still nothing after four then I would resow with new seed.