Picking off the grubs can work well but in reality taking the time to do this at our place doesn't seem to happen! It also is a continuous job as the butterflies will continue to lay eggs on the plants.
We have found it is actually very simple to keep grubs out by placing a frame with bird netting over your brassicas which acts as a barrier and the cabbage white butterflies can't get in to lay eggs...hence no grubs. This is the simplest and most successful method we have used.
Another idea we tried was placing a white golf ball in the tunnel house we planted our brassicas in last winter. This theory is based on the fact that the cabbage white butterflies are territorial and they will see the white of the golf ball and think it is another one of them and go elsewhere to lay their eggs. This didn't completely eliminate the grubs but they were certainly not a huge issue.
I found an interesting article yesterday where instead of a golf ball you print out pictures of the cabbage white butterflies and laminate them and place them around your brassicas. I am curious to try this and see how it works. Here is a link for you if you want to give this a try.