Instantaneous water heaters sound like the perfect water heater - limitless hot water on demand, no wait time, no water stored, compact and easy to install.
All of this is true, but there are also some limitations, such as the number of simultaneous outlets, proximity to the outlet, and the need for its own cable from the consumer unit. Most instant or on-demand units are purpose-built to serve 1,2, or 3 basins, but there are some just about powerful enough to handle a shower. At single-phase power, there are solutions for everything up to but not including a bath - please read here for more on that.
To make sure they will fit the requirement you have in mind, why not run through the Product Advisor to help you choose the most appropriate appliance by clicking on the image below: