I think a risk assessment to see if the race can go ahead would suffice.... I stand corrected but I'm sure RO's must do some form of risk assessment for insurances purposes.

Obviously alongside some restrictions/guidelines as decided by the powers that be)

For example perhaps a race like Borrowdale couldn't go ahead this year, with several hundred runners all charging to a gridlock within 200 yards.

But for example Edale Skyline where the start line is quite well spread, and it's a good uphill run until I recall any kind of bottleneck, could go ahead.