Just yesterday we wrote that LG is finally offering a way to unlock the bootloader on the LG G6 (H870). The community didn't need much time to port a custom recovery.
Thanks to the efforts of the XDA Senior Member nima0003 and XDA Recognized Developer Rashed97, TWRP is available for the European variant of LG G6. The recovery allows users to flash the root package, kernels, mods and of course custom ROMs to replace the stock firmware. Please follow the instructions in the thread to get it working.