Marshmallow ROM for HTC One M8 (UK)

[AC Question]

Just bought a HTC M8 from China..took a while to arrive but all is good and works great and very fast.. my question is: the software version on it is 4.4.4 which i believe is KitKat.. when searching for new software updates it finds nothing, i thought it would at least update to Lollipop 5.0 so i'm wanting to manually push the update into the phone and update that way if possible? The phone is S-On in the bootloader so i know i need to get Sunshine S-Off to get that sorted but thats about all i do know - i have flashed the M7 a few times with custom ROMs but that was a while ago and forget how to do it - If anyone can help or point me in the right direction to where i can get the version 6.0 update ROM or a good custom one i'd be much appreciative.. thank you

HTC One M8
Bootloader - LOCKED
S-ON
HBOOT 3.19.0.0000