Why does my bmw 320d 02 plate keep going in to limp mode when launching off start but when I start off slowly and then launch it works fine. I have had it plugged in and it come up low air pressure.

It sounds like you have an intake or boost leak of some sort. I assume the 320d is a turbo diesel. You will likely need to get someone with a smoke machine to try and find the air leaks. This makes sense since you mentioned you can't "launch" at higher RPM which sounds like the boost that the turbo is making is leaking before getting to the throttle bodies and cylinders.