You book a person rather than a company. The booking form knows which of the four does which work, so the times you are offered for an electrical visit are the times the electrician is genuinely free.
It also means a returning customer can ask for whoever came last time, which matters more than we expected. Half the value of a second visit is the tradesman remembering what is behind the cupboard.
When somebody is on leave or off the island, the form simply shows fewer times rather than offering you a slot nobody can keep. A short list of real times beats a long list of hopeful ones.