What?
You have insurance so that they pay out if you cause an accident.
Insurance companies want to keep costs down, so if you are the injured party and say to them "give me a courtesy car while you sort out fixing mine" then they'll be very happy, as it keeps their costs, and, therefore, our premiums, down.
You could also organise your own courtesy car and repairs and then bill the other company for it, which is the most hassle for you. You could also contact Helphire or other such company, but all they do is what their insurers would do, only they charge a nice big fee for it, increasing costs.
if your insurance are doing a job like Helphire would (i.e. arranging), only for no fee then fine, but you certainly don't want to be claiming off your own insurance for a non-fault accident.