As I said, this all relies on the length of cycle times being longer than the time needed to get other measures "fired up". Immersion heaters have cycle times of at least several minutes, as do (IME) fan heaters - I'd say at least an order of magnitude longer than it takes to (for example) open the taps at Dinorwig or call up some diesel gennys.
I thought the 'taps' at Dinorwig took just minutes to bring it online?
Likely less time than it would take to select and signal sub-stations to change their tappings to reduce voltages. I agree with JohnW2's suggestion that reducing voltage would only have a limited short term effect, a matter of minutes to perhaps an hour. Spread over several hours the Mwh demand would be identical, as consumers would make up for reduced voltage.