Fixed immersion water heaters 15 litres should be powered by their own
separate circuits.

Not to my knowledge. However, you may not apply diversity, so a 3kW @ 240V
heater needs to have 12.5A of a circuit dedicated to only its requirements
that can't be borrowed by another device. This would preclude it from being
installed on a ring main or socket radial circuit. However, a 16A radial
circuit with a 13A FCU to the immersion and a 3A FCU to the pump is fine.

Christian.