I am a Britisher and I have never heard anyone pronounce SEGA as SEE-GA. It has always been pronounced SÉ-GA to me (it even informs you how to pronounce it at startup in some games). Also Konami is pronounced as it should be KO-NA-ME.
Your information source is unreliable.
My and my friend used to pronounce it SEE-GA. To us, the megadrive was the '16-bit SEE-GA' for a while. Then, we corrected ourselves once we knew better i.e. after there were actually speech samples in the games.
I used to call a certain game "Cheenah Warrior" because a friend pronounced it that way and I assumed he knew something that I didn't. Note: He was a dumbass, but that still didn't raise any red flags. I didn't actually own TG-16 yet, let alone the game. We said "Cheenah" for a few months (between console launch and XMAS, when I finally bought TG-16).
I am a semi-intelligent person. I still don't understand why it took me so long to realize the obvious.
I would love to tell you that I was I intentionally applying a Spanish pronunciation to an obviously English word...but I wasn't.
My dad had really bad sleep apnea and he would gasp for breath in the middle of the night. It sounded just like: "SEEEGAH!"
I am pretty certain he was an agent of FEKA.