unlock codes for Voda are £10 on ebay, item number: 122090398483
you can put any stock rom on the s7, it wont trip knox (as long as its official Samsung rom)
http://www.sammobile.com/firmwares/database/SM-G930F/BTU/
loads of guides on youtube, but its very easy using Odin
Code:
Instructions
Extract (unzip) the firmware file
Download Odin v3.11.1
Extract Odin ZIP file
Open Odin v3.11.1
Reboot Phone in Download Mode (press and hold Home + Power + Volume Down buttons)
Connect phone and wait until you get a blue sign in Odin
Add the firmware file to AP / PDA
Make sure re-partition is NOT ticked
Click the start button, sit back and wait few minutes