If the UK stopped growing oilseed rape, the grains would still be imported from abroad. Last year 200,000 Tonnes of oilseed rape was imported from Australia, where these chemicals (and others), banned in the EU, are still permitted. So in effect we would be exporting our environmental conscious abroad.
We haven't sprayed any insecticide on the oilseed rape crops this year. Partly because we are trying to farm without using insecticides as we want to encourage our beneficial insects to predate the pests and partly due to the fact that I am not sure exactly how efficient they are due to pest resistance. It seems that they are becoming resistant to the active ingredient so their extended use is probably not justified.
Either way new research is needed to find out more about the pest and how we can manage or disrupt its life cycle. What percentage of the population is resistant? What environmental trigger makes them hatch? How long is the feeding period for?
These are some of the issues faced on the farm by one crop and one pest. We have a massive challenge to maintain active ingredients that enable us to continue to grow food in the UK. Food sovereignty should be very much at the fore front of the thinking of the population in a post EU world but we will need to tools that will unable us to product this food for an ever increasing population.