Not sure about monthly payment options (think they do 3 or 4 instalment plans), but we insured both of our sons through Aviva (eldest is 24 and drives a 1.2 Nissan Micra, youngest is 19 and drives a 1.2 Renault Clio).
Cost for each is well below what we were expecting and when eldest was involved in a shunt (hit from behind while he was stationary), Aviva were brill in helping sort everything.