[G4 Q&A, Help & Troubleshooting] Yuuseki:
The 10X version are all Lollipop, 20X is marshmallow.

When flashing a KDZ on your phone, what's important is what model your phone is. H815 in your case.

If you want to restore to stock in case something goes wrong, you can find everything here: http://xdaforums.com/g4/general/guide-lg-g4-stock-firmware-to-stock-kdz-t3107848

In there is a link to this directory: http://storagecow.eu/index.php?dir=Xda/LG+G4/ where you find pretty much all KDZ files for G4 phones.

I'm not that experienced myself, but I rooted my phone just recently and I can share my experience with you.

I have a H815. It was a V10c-TWN-XX version (Taiwan). I updated to Marshmallow with LG Bridge also and the new software version was V20b-TWN-XX.

Marshmallow rooting is not possible on my phone because I can't unlock the bootloader on it. And Marshmallow rooting is only possible with an unlocked bootloader. Unlocking the bootloader is only possible on European phones (http://developer.lge.com/resource/mobile/RetrieveBootloader.dev?categoryTypeCode=ANRS). I asked autoprime (renown user and developer here) if I can unlock bootloader after flashing an European KDZ on it. He said no, having European software on it is not enough, it has to be a phone manufactured for the european market. Dont know why, guess because of different hardware.

So, not being able to root MM, I had to revert back to stock (downgrade) using one of the KDZs.

I chose the most recent one in the directory I linked (which is the Poland H81510E). In the "Go Back to Stock" thread I linked the OP says to use LG Flash Tool 2014, which unfortunately doesn't work with Marshmallow. But fret not, it works with LG UP! http://xdaforums.com/g4/help/lgup-install-kdz-mm-lp-t3249803

When I tried flashing the KDZ with Flash Tool 2014, the process failed with an error at 5%. It worked flawlessly with LG UP. Here's more information regarding downgrading, but it really isnt hard. Just download + install LG UP. Launch it. Connect your phone. Choose KDZ file. Select option "upgrade". Hit start.

NOTE: after the firmware update was done, the phone rebooted automatically and for me it stuck at at the LG Logo (bootloop). But a simple hard factory reset did it, the phone booted and my new version was 10e-something. With Lollipop 5.1 of course.

After that, I rooted my phone using this pre-rooted image: http://xdaforums.com/g4/development/h815-eu-l-p-t-sea-twn-t3176195

and this guide: http://xdaforums.com/g4/orig-development/root-tmo-vzw-intl-variants-soon-root-lg-t3164765/

I now have a "H81510e-EUR-XX" Lollipop 5.1 with root (SuperSU 2.49).

I don't really know about Vodafone and if carrier specific firmware is necessary in this case. If not, you could use any 815 firmware.

From the site you have linked with the KDZ files..... it seems they are all the same.

If you check the table for H815 KDZs from Vodafone, there are some 10C and 10D versions. Now I compared the fileKey parameter in the URL to the 10C files and the 10D files and also the bytes of the files.

All the 10C version are identical to each other, same for 10D.

Check out this screenshot I made for you: http://i.imgur.com/Ie0w4us.png

Bytes are also identical, so it's pretty safe to say those are the same files.

Dont know why they are listed several times there. The suffix doesnt matter.

I'd grab a 10D if I were you. But I think the Poland 10E should work, too. Better confirm it with somebody else though.

If you flash your system to 10e, you can use the pre-rooted image I used. Otherwise you will have to inject root yourself: http://xdaforums.com/g4/general/lg-g4-100-root-success-directives-root-t3180586. In this method you create an image of your system and inject root into the image, which you then flash back on your phone.