Made by Pulsar Advanced Technologies, these are a very new technology and aren’t as yet widely available, but microwave boilers have all the hallmarks of the perfect solution.
Small and lightweight
Instantly heats water on demand
Capable of supplying taps, showers and appliances simultaneously
Does not require a hot water tank
Does not require a mains gas supply; uses electricity instead
Energy efficient
Cost to purchase as yet unknown
Should be simple to fit, but few tradesmen will yet be experienced