[Q] Downgrading from 5.0 back to 4.4.2 UK topic

Hey all,
I have tried searching about this on the forum, but I can't seem to find anything solid. And I'm a novice at that.

I made the mistake of auto updating my S5 to Lollipop and my phone is seriously laggy, wont respond at times battery drains very quickly etc etc.
I've looked into ways of downgrading back to 4.4.2 and I'm sure I can use Odin and possibly Kies.

Kies seems like the easier option, can you downgrade using it? Through Tools, then Firmware Upgrade and initialization? Or will it restore it to the current version 5.0?

Odin seems to be more difficult, I've seen comments on how it hasn't worked out and I don't want to muck my phone up I've only had it this year, this procedure wont void the warranty will it?
Going through SamMobile on country, my carrier is o2UK would I select o2 or BTU? As I'm sure my phone shipped unlocked. (bought it through o2)
Would I need to also download a 7Zip driver? Or use the stock one which comes with Windows?


I've backed the phone up using Kies3.0, I'm just scared about using Odin.

This may seem ridiculous to you, but I would really appreciate any help, or tips.
Thanks.