I totally agree on the logistics side of things - looking at it that way, there's probably no other decision you could make.
I'm still cautiously optimistic that we'll see a busy Autumn for racing. I think you can make some form of distancing work during a long race if necessary, but it's possible that testing capacity will be at a point by then that we can make sure it's safe with or without social distancing.
When you balance that with the mental / physical health benefits that come from encouraging these kind of events, and the significant amounts of money it raises for charity, I think there needs to be a concerted effort to make as many of these races happen as possible.
None of that is intended as a criticism of the Hatters Half organisers. I'd make the same decision.