From a public health standpoint it makes more sense to have two hospitals with 100 beds at 90% capacity than one hospital with 300 beds at 60% capacity. It's leads to better patient outcomes to have the hospital staff work overtime compared to rapidly hiring and training new staff whilst the hospital is being overrun.

Why ? What’s the rationale ? How they arrived at 90 percent figure ?

Do you have a source for that quite extraordinary claim?

Not the parent, and not a backup for the specific claim, but I know that Tory governments since Thatcher have been taking the strategy of misapplying business efficiency tactics to NHS, then when they don't make sense and cause worse outcomes claim that it's because government was involved at all and call for privatization.