Seems to vary between OPCs, even in the same group (Dick Lovett). The last three loaners from Swindon have been a Toyota and a couple of BMWs. So far, including yesterday, I’ve always had a Porsche courtesy car from Tewkesbury.
I just try and book well ahead. At short notice, I guess that any OPC may struggle to produce a loaner. Some OPCs may have reduced their courtesy car fleet to re-stock their used cars for sale.