Reverting back to stock for Warranty Purposes

Need to send back my Z3 compact for some warranty issues.
What is the best way for me to revert to stock?
I already have all my apps backed up and a twrp backup as well.

I am rocking a modified rom, and want to make sure I don't meet problems due to it. My bootloader is not unlocked.
Also I ordered my phone from the UK from Handcent Is there a specific firmware I should use then?
What do i need to do to revert back to complete stock?

Thanks for any help!