Hi and welcome. To answer your questions, first off, rooting and flashing custom ROMs is a little more involved on an HTC compared to most other Androids, in particular phones like Nexus (Google) devices and Samsung's range, all of which a fairly simple to root and flash with custom ROMs.

First of, whatever you plan on doing you need to unlock the bootloader of your HTC One via the HTC Developer website (link here https://www.htcdev.com/).

Click on the 'Unlock Bootloader' icon and follow the instructions to the letter (when choosing your device choose not listed, most recent phones are not listed).

Once you've unlocked the bootloader you can flash a custom recovery, followed by a custom ROM, such as the Venom. However, the initial steps to flash a custom recovery are all done via the android debug bridge environment (adb), which is a physical connection (USB) between your PC and your HTC One, the latter has to have USB debugging enabled in 'Developer Options'.

I strongly advise to read as much as you can on the subject of rooting and flashing custom ROMs and firmware updates and how the bootloader (hboot) version influences any choice of ROM and more to the point, the firmware. You can find out all you need on this forums 'all things root' section, or on the XDA forum here HTC One - xda-developers

There is also another important item, the so called S-status, i.e. S-On or S-Off. Out of the box all HTC devices are S-On, which is a security flag in the bootloader that prohibits certain flash attempts that try to change system critical elements of a partition, such as kernel flashes and of course bootloader changes.

In order to make changes to these system elements your phone has to have the S-Off status. However, if you just want to change the recovery and flash a custom ROM this is strictly not necessary, but if you ever make a mistake, say you flashed a ROM which was not intended for your device, i.e. a different CID code, you might not be able to recover your device if it is S-On, whereas if it is S-Off this will be possible.

Another thing that is crucial is to get the correct and properly working USB drivers for your PC environment, which can be quite a challenge at times, but you will find alternatives to get it going; if you have a Windows PC then I'd recommend Kouch's Universal Windows Driver Setup, which was the only package that worked for my Windows 8.1 setup and can be found here https://github.com/koush/UniversalAdbDriver

To summarise, my recommendation for your proceedings would be as follows:

1. Unlock bootloader
2. Install correct USB drivers and check that your phone is recognised properly by returning your device Serial number in adb
3. Check your device info (adb - type without quotes 'fastboot getvar all'
4. Note down your CID code (in case you want to return to stock), hboot version (for compatible firmware, if you wish to update it)
5. Get your device S-Off, although not strictly necessary, but highly recommended (the only way to remove the 'tampered' flag for example is to be S-Off, but this is not the only benefit)
6. flash custom recovery and root

All of this will keep you busy for a while. As mentioned at the beginning, read as much on the subject as possible and don't pay too much attention to some indifferent YouTube videos.

 

No worries. first of all, to get items listed in my post 1. to 6. achieved you can read through and follow the guide in this link [Guide] Vomer's Ultimate AIO - S-OFF, SuperCID, Firmware Upgrade & Custom Recovery - xda-developers, which I used when I started to root and S-Off my ONE a couple of months ago. You might also follow some links in the Ops post as they are very useful.

As far as finding a suitable RUU is concerned I must confess there aren't many for 3 UK, but I personally don't see the need to return to a 3 stock/shipped ROM as with any warranty job you would be better advised to contact HTC directly. All you'd have to do is change the CID to whatever variant you intend to flash, e.g. HTC Europe, which would be an (Carrier) unbranded stock HTC ROM.

You can also flash some Nandroid backups of entire stock ROMs that some people have taken and made available and amongst those I found a 3 UK one here [COLLECTION] HTC M7 RUU, OTA, and Stock Nandroid Downloads - xda-developers - it is a CWM recovery based backup so you'd have to have a CWM recovery installed (personally I prefer TWRP).

And here are some links to other sites that host RUU ROMs -
http://www.shipped-roms.com/index.php?category=android&model=HTC One
Collection of HTC One (M7) RUU File Downloads. Reset device to Stock

Finally, just make sure you get ROMs and firmware for the ONE M7 UL version, otherwise you might end up with an expensive paperweight.

I am afraid for me to rewrite everything that is already out there and for which I have provided relevant links, would be silly and might be even more confusing. There is no substitute or a quick fix for researching quite a bit yourself, except using peoples advice like mine as a guide.

P.S. here is another useful link with relevant explanations you should read to avoid potential disaster - http://forum.xda-developers.com/showthread.php?t=2182823

 

Being 'Off line' suggests that there is no communication between your phone and the PC (Mac). I'm sorry, I am a life-long Windows man and hence can't really help you with any Mac related issues.

Sounds like a driver issue. Alternative would be to find a Windows machine and use that temporarily until you get sorted; just a thought and probably not what you want, sorry.

 

Another thought, open/run the sdk.exe from the install folder and update any of the packages on offer for update and ensure that the USB driver section is enabled and up-to-date.

 

A lot of times a simple unplug/replug will get the device online,try different ports,and a reboot of the machine and phone. Also make sure you have turned on developer options and enabled USB debugging.

Fear not,sprint and vzw are the only variants that will take different files. Your Three UK device is gsm,and anything that's designed for gsm will work. European,us developer,att,t mobile,etc all can use the same guides and files.

 

the VERY FIRST thing you should do is make a backup of your unrooted stock rom(or rooted if youve allready installed superuser) and copy it to your PC for safe keeping. it may be valuable later,if you need to return to stock.

also keep a copy on your sd card,so you dont find yourself making a "im stuck in recovery with no OS and dont have a backup or rom on my SD card" post. weve seen that many times,and having a backup present could save you alot of headache

after that,you can download the rom you want to use,transfer it to the phone,and perform a "wipe data/factory reset" and then flash the new rom.

 

you sure can. just do the wipe data/facotry reset and youre good to go to flash the new rom. if you want to go back to viper,just do the wipe data/fctory reset and then restore that backup(or reflash the rom)

hope that helps