There is no such requirement made in either planning legislation or through building regulations. However, s.17 of the Crime and Disorder Act 1998 places a statutory duty on a local authority when exercising its functions to have regard to the need to do all it reasonably can to prevent crime and disorder in its area.
For planning purposes, guidance has been issued ‘Safer Places: The Planning System and Crime Prevention’, that includes advice on making it more difficult for criminals to commit offences. While letter boxes are not a planning matter, the guidance is still relevant as it talks about complementary measures that can be taken.
Planning Policy Statement 3 Housing advises how local planning authorities should draw on a range of guidance and standards to facilitate efficient delivery of high quality development, including the guidance mentioned.