The electoral register is not a stable or reliable basis to use as a way of allocating constituencies, because the numbers of people on the register has varied wildly over the transition period – from 45.3m in December 2014 up to 46.4m in May 2015 and then down again to 44.7m in December 2015. If the registers get a lot better over the next few years, as advocates of Individual Electoral Registration suggest they should, then the result will be hugely oversized urban constituencies. The Census suggests that if everyone who was entitled to be on the register actually was, there would be a little over 40m on the register in England rather than 37.6m.
Fortunately, there is a better way. The Census itself could be used as the basis of drawing constituency boundaries; either to model what a complete register would look like, or to base representation on the principle that is most commonly found in the rest of the world, from Ireland to the United States – that MPs should have roughly the same number of constituents, including children and foreigners. Boundary changes would be every ten years, rather than every five-year parliament as the current law requires.