require hard-wiring and citing in and around the consumer unit, hence why some installers will not touch these products with a barge pole.
Don’t understand the problem. Installation in that position is really simple unless there is no space there. PV installers are usually wiring in that region so adding a diverter is a trivial extra task. A totally plug in solution using 13 amp plugs might be easier but few immersions uses 13A plugs so hard wiring is needed. I’d not trust a 13 plug/socket to pass 13A reliably long term so I’d not supply such a product. E.g. how do you know the customer's socket is not in poor shape?
Current distortion is a problem for the DNOs because of all the many switched mode supplies (SMPS) in service. SMPS tend to consume most of their current in the 60-100 degrees phase angle region as per:-
http://www.ieee.li/pdf/viewgraphs/harmonics.pdf Section 1.4
Power factor - Wikipedia, the free encyclopedia Section Non-sinusoidal components
which show a short triangular current spike centred on 70-80 degrees
A phase angle controller, when at other than full or zero power, on average consumes its power later than 90 degrees and it’s not as spiky as for an SMPS, unless at low powers (>120 degrees) when the spikes are not that high anyway. Phase fired controllers - Wikipedia, the free encyclopedia So it’s filling in the gap in the last half of the SMPS’s cycle.
Therefore if I were a DNO I’d quite welcome some phase controlled devices onto the supply to compensate for SMPSs or at least be ambivalent about it. It’s not perfect compensation and so I’d not recommend a street full of phase controlled diversion boxes, but I don’t see a problem with a few. There is no adverse effect on the rest of the electrical devices in a property or in its neighbours, unlike with burst firing, as long as a decent EMC filter is used. It’s only the DNO that might notice some distortion if there were too many of them.
A PWM device is one solution but they will be expensive and you have to have a screened wire between the device and the immersion due to the very high switching speed of the PWM that causes this wire to radiate EMI. In my house it is well nigh impossible to access this wire to change it and in almost any house this will involve significant work. So it will add a lot of cost.