[Q] What version do I have to use? topic

I've recently bought the lg g3 unlocked from Amazon in the UK. I updated it to official lollipop not knowing it would unable me to root it. Now I'm looking to downgrade to kit kat in order to root it with purpledrake, install TWRP and the Cyanogenmod 12. My problem is I don't know exactly what firmware to install. The only information I have about the phone is that it is the D855 16GB and that its software version is "V20f-EUR-XX". If some one could point out the kit kat version to be installed and any problem with my plan I would be very grateful. Thank you