The only Airfix kit I have seen is of the 250LM, not the "P" car.
There was a rtr in the 60s that I have the body of, but not seen the body or kit separately(an english collecter who has since died got them for me).
As for the MRRC kit. I am in america. I have NEVER seen any of the MRRC kits except as a listing. And a few photos. If I understand the photos, it seems that the kits are nice and about 3 times the cost of the RTR, using a revell type aluminum chassis, and metal parts everywhere. The few times I have hears of a U.S. supplier getting any of them, they seem to sell out before I get a shot!
Anyway, in the day, I only briefly ran a Mono P car. Atlas had a conversion kit to use their 208 powered car that I liked better, and I have scratchbuilds. I love the car. Besides the ones you mention, there are versions that ran in Red with a white nose, or white stripe or light blue panels. There was a car that ran a bit later that was blue and white. This is a tiny car and fun to build things under.