How to fix phone booting only into fastboot mode after flashing a custom ROM?
Old thread, but helped me fix my Wileyfox Swift as well. 

I had the same issue - followed the TWRP route, erased and loaded the new image.
On restart, the bootloader refused to load the system, just went round and round. Speculation on my part, but this might have been because some of us had to "patch" the 'fox bootloader to get the Android 7 update to load in the first place.

The links on the facebook page posted by Bo Lawson (thank you) are still active at the time of this post. I selected the "last" cyanogen release (see the Swift links, used ZNH2KAS7EB).

TWRP was already on, so I booted to recovery, used adp to push the file SW27-WF-CRACKLING-CM-13.1.5-ZNH2KAS7EB-RECOVERY.zip to /sdcard, then used TWRP to install it. 
This also replaced the "new" bootloader. The old Cyanogen image booted without issues, so just turned on USB debugging, then rebooted to bootloader.

Fastboot flash'd recovery with TWRP(3.2.3.0) again. Rebooted to recovery, then installed the replacement image (not Lineage, but e.Foundation's beta).

Works!
...and there was much rejoicing.