Wow, that technician was completely useless. He took your £70 and did absolutely nothing. At the very least, he should have at least updated your motherboard BIOS for you for that kind of fee.
You keep on going back and forth on this BIOS issue while doing absolutely nothing to determine whether or not that's actually the issue. It's not your fault. Everyone you're reached out to so far for help has been worthless.
Start by actually updating your motherboard BIOS to the latest version. That will at least remove the BIOS from the equation as the potential issue. Read your motherboard manual. Specifically, look up how to use the "Q-Flash Plus" feature on Gigabyte motherboards. You download the latest BIOS from Gigabyte's website, put it on a USB drive, and use the Q-Flash feature to update the motherboard's BIOS if you don't have a working CPU or RAM installed.
Once you are on BIOS F4d, you can figure out whether your CPU actually boots or not.