You're asking online how YOUR installation is set up. That's a bit worrying frankly as we cannot see into your house!
Based on the information you have give, I simply cannot tell you what the tank is doing. What I can tell you is that MOST electric showers work on mains pressure water from the mains supply pipe and not from pipes coming from water stored in a tank. So whatever the tank feeds, it probably does not feed the shower and you probably will be fine with isolating at the mains.
But you really do need to check for yourself that the water is isolated before disconnecting the existing shower fully. If you are unable to check this for yourself, either get a professional in, or go ahead with the DIY aware that you are taking a risk and it may end badly. I wouldn't want to put you off, but please do make sure you aware of the potential risks and have a backup plan in case something does not go quite as you expect it to.