You can get space saving cold water tanks, you can relocate the tank into the eaves out of the way for the loft conversion, leave you dhw tank in the airing cupboard and convert the heating to a sealed system.
Fit a water pump so you get high pressure to all taps and showers (I presume all the showers are thermostatic mixer showers) and that will/should supply enough for your needs. In a house your size a combi will not be the right choice.
I don't know if realistically you will get 20ltrs a minute flow rate