Hi Lowermeadow,
It is sad when you lose your carefully nurtured plants to something like this. I've suffered the same as you with cabbage fly in the past; this year for me it's woodpigeons which haven't previously been a problem, I'm on my 3rd sowing of peas, now behind metal fencing!
The good news is that the collars do work (search online for "brassica collars"), and it's worth putting them around the remaining healthy plants. They won't work if the grubs are already there underground on the healthy looking plants, but they will protect against further attacks. Cabbage fly comes in 2 or 3 waves over the season, and it's this first wave that normally does the most damage because the plants are smaller.
In terms of understanding what's happened, it's often the case that the first time you grow something new you get good results, and then subsequent crops aren't quite as good. It's partly because the first time you grow something the particular nutrients that crop needs haven't been depleted, and partly because any pests specific to that crop are small in number and you don't notice any damage. In second and subsequent years however, the pests have built up in the previous year. Rotation and manuring help with nutrient levels, and rotation also helps with soil borne diseases but can only help a bit with a fly which can still move around your garden and find host plants nearby.
G&R