I wonder what recent event governments could have used to show that this was a reasonable course of action to take. One that they could manage easily when it is in its early stages, having been warned about it. Perhaps one that affects humnans?
What thing could they have managed better that would make us trust our government in the UK to be using this known bee killer to manage a disease for a greater good later on?
Regarding "we can just import it", well yes, but if everyone took that opinion then there wouldn't be any farming anywhere eventually.
don't know if you should trust this link, but it's a decent paper: https://ageconsearch.umn.edu/record/7000/files/cp02zi02.pdf
p.s. The paper ends in the late 90s although little has changed other than the closing of the US cane industry in Hawaii.
Application to sugar beets is expected to increase crop yields 13% on a non-critical crop, for a commodity (sugar) that can be easily imported. The virus problem won't ever go away. So this just sounds like subsidizing a particular local agricultural industry, because it's having a hard time competing right now. Will that ever change, why not choose bees or wildflowers over sugar beet farmers? Is it just because they can pay for lobbyists or positive news coverage?
Furthermore, it sounds an awful lot like the massive use of (the most advanced) anti-biotics in cattle feed lots to increase meat yield. That builds up immunity so that those last line of defense drugs are no longer useful to treat humans with deadly diseases. The key here is that there will be a state of permanent emergency where some new poison is always needed to keep profits at their previous artificially high level.
If the local industry can't compete, and it's not necessary other than for a small group's continued profit, maybe it's time to move on. The alternative is a possibly long term loss of pollinators (because the insecticide goes down streams) and the development of stronger/larger reservoirs of mites, moths, molds that could permanently affect all pollinators in England.