S A Plumbing & Heating
Flow Boiler

Flow boilers
These used to be called 'electric boilers', but now the manufacturers seem to want us to call them 'flow boilers' for some reason.
Fflow boilers are little more than an immersion heater in a pipe. They have some sophisticated electronics in them.
They were developed and marketed originally by a company called Amptec, from Basingstoke, to allow a wet central heating with normal radiators to be powered by electricity. The trouble is, this is normal, day rate (i.e. EXPENSIVE) electricity. Amptec made some false claims about low running costs and were driven into insolvency by a series of customers successfully suing them, and the company has now been taken over by HeatRae Sadia.
HeatRae Sadia are planning a major marketing push for Amptec boilers again now, but because of the huge running costs I expect them to remain a specialist niche product. I've never seen one fitted in a real dwelling.
See HeatRae Sadia's Amptec page here 
Trianco (the oil boiler manufacturer, not the model train people) also make two electric boilers, both called "Aztec". See them Here.