When?
1 January 1997 (for young people under the age of 16) and 1 October 2007 (for those aged 16 or 17).
[Section 141A Criminal Justice Act 1988, as amended by Section 6 of the The Offensive Weapons Act 1996 and Section 66 of the Violent Crime Reduction Act 2006].
However that assumes that a hacksaw is regarded as a 'knife', so it seems that that Maplin's legal advisers are possibly being rather over-cautious. (The legislation only applies to "(a) any knife, knife blade or razor blade, (b) any axe, and (c) any other article which has a blade or which is sharply pointed and which is made or adapted for use for causing injury to the person.")