How can I downgrade my S6 from Android 6 to 5.1.1? Errors using Odin.

[AC Question]

Everywhere I look the idea seems to be to use odin and get the appropriate firmware from SamMobile so I put in the details of the phone and downloaded this piece of firmware:
G920FXXU2BOG8_G920FBTU2BOG9_BTU

The phone is:
- model G920F
- a UK phone
- bought sim free unlocked

When attempting to flash the firmware onto the phone using odin it fails very early on. See the output in odin below:
<OSM> Enter CS for MD5..
<OSM> Binary Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/005> Added!!
<ID:0/005> Odin engine v(ID:3.1100)..
<ID:0/005> File analysis..
<ID:0/005> SetupConnection..
<ID:0/005> Initialzation..
<ID:0/005> Get PIT for mapping..
<ID:0/005> Firmware update start..
<ID:0/005> SingleDownload.
<ID:0/005> sboot.bin
<ID:0/005> NAND Write Start!!
<ID:0/005> FAIL!
<ID:0/005>
<ID:0/005> Complete(Write) operation failed.
<OSM> All threads completed. (succeed 0 / failed 1)

And on the phone I get the following:
SW REV. CHECK FAIL. BINARY: 3, DEVICE: 2

The only suggestions I can find is that I am picking the wrong firmware, so I tried a couple others for the same model of phone but no luck.

What should I try next?