The information requested falls within the responsibility of the UK Statistics Authority. I have asked the authority to reply.
Letter from Jil Matheson, dated November 2009:
As National Statistician I have been asked to reply to your Parliamentary Question asking what the most recent estimate is of the number of people eligible to register to vote in (a) local, (b) European and (c) UK Parliamentary elections. (299166)
ONS publishes the number of people registered to vote in local, European and Parliamentary elections but does not hold data on the number who are eligible to register. The mid-year estimate of the resident population, aged 18 and over, provides an approximation to this although not everybody who is usually resident is entitled to vote. For example, foreign citizens from outside of the EU and Commonwealth are not eligible.
The definition of the usually resident population is closer to the eligibility criteria for the local and European electorate than to those for the parliamentary electorate because the latter exclude EU citizens and include UK citizens resident abroad.
The mid-year estimate of the resident population aged 18 and over for the UK, for 2008 is 48,263,000. This is the most recent year for which estimates are available.