How is it strange that only British citizens can vote in British elections?

Michael Crick has just blogged about a ‘strange quirk in the British political system.’ Writing about a Lib Dem councillor he says ‘interestingly, because she is a Danish citizen, double-councillor Lockington isn’t just disqualified from standing as a candidate in the coming general election, she can’t even vote in it.’ But what seems strange to me is the idea that people who aren’t British citizens should be able to vote in a British general election.

PS I should, of course, point out that Irish citizens can vote in UK elections because of a reciprocal agreement made at the time of the foundation of the Irish Republic, which the Irish then reneged on. Commonwealth citizens resident in the UK can also vote.