You can use a combi as a syѕtem boiler. In most homes the only high pressure outlet you need is the shower. Have this off the combi from the cold water mains. Fit a `combination cylinder`. That is a hot water water cylinder and cold water tank in one unit. It saves a lot of space. All other hot outlets can be off this. Take the cold taps off this as well. You can get some with a large cold tank section. It operates on low pressure and is far better than an unvented cylinder. It gives you stored water and the shower is not affected when the dishwasher kicks in.
If you have a small cold tank section, fit in a 3/4" ballcock and 22mm cold feed to it. It will fill fast.
If you have a small cold tank section, fit in a 3/4" ballcock and 22mm cold feed to it. It will fill fast.