In 2005 a Commons Select Committee published a detailed study into electoral registration. Chapter Three on "Encouraging Registration" begins with consideration of "the problem of non-registration". It is worth reading - whether your interest is in politics or the functioning of the legal system.
The report states "The most recent estimate we have is that in 1991, 93% of those eligible to vote in Britain were registered, but beneath this headline figure, there is a more worrying picture where “non-registration rates appear to vary by geographical area, by age, ethnicity and property ownership/tenure”. Of course, several factors will apply in some individual cases but in general those less likely to be registered are: men; those living in London; those living in urban areas and areas of economic deprivation; those aged 17 to 24; those in privately rented accommodation; and those from black and minority ethnic communities."