Gravity fed mains unvented hot water cylinder is this possible

Hi All, I am trying to future proof my new installation, and supply a unvented mains pressure hot water cylinder with gravity stored water approx 1.0m head if fitted in one location 2.5m if in a different location my water mains pressure is not great currently. I am build an extension with three bedrooms and shower rooms, it’s possible all three showers will be used at once .
so last resort is to fit a gravity hot water system and use individual silent running wall mounted triton shower pumps in each bathroom.
I am considering upgrading my cold water mains supply but I will have no idea if the new supply will run 3 showers at once . Hence trying to find out if using an unvented hot water cylinder with a gravity supply that I may change over to mains supply in the future is posdible. I know I will have to change the wall pumped showers to shower valve if I go mains pressure .

i am think of fitting a 300lt cylinder .

hope this makes sense c

thanks 

Jeff