Can't get M8 back to stock? Can't Find the right ROM?

I've been trying to turn my M8 back to box factory settings with little luck. Most of the guides I've seen say you should relock your bootloader (done) then run RUU with stock room.

The main issue I have with this is that I have no idea what ROM to use. My phone carrier is Three (United Kingdom).

I've tried following this guide HERE but I get this error:

Code:

---

D:\Users\Chris\Desktop\RUU>htc_fastboot flash zip RUU.zip
sending 'zip'... (53965 KB) OKAY
sending time = 2.472 secs
writing 'zip'... (bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
FAIL42 custom id check fail
FAILED (remote: 42 custom id check fail)
Execution time is 6(s)

---

Can anyone help? I've followed like 5 different methods, but most of the time I am not sure what version of file I need as my phone carrier is Three (United Kingdom) and isn't normally listed.

Here is what my FastBoot screen says:

Code:

---

M8_UL PVT SHIP S-ON
HBOOT-3.19.0.0000
RADIO-1.25.21331147A1.06G
OPENDSP-v48.2.2-0564-m8974_f0.1211
OS-
eMMC-boot 2048MB
JAN 16 2015, 18:54:06.0

---

I do not think I have TWRP anymore because when I use Quickboot app to boot into recovery it just takes me into the Fastboot version rather than TWRP.

Thanks