This "Ninja sword" is apparently a straight sword:

2.4. The legal definition that we tested as part of this consultation was as follows:

A fixed bladed article with a blade between 14-24 inches (the length of the blade being the straight-line distance from the top of the handle to the tip of the blade) with:

(i) A single straight cutting edge; and;

It sounds like most katanas and such, what I think people probably normally think of when they think of a "ninja sword", are already banned under existing legislation, as they're curved-edge, rather than straight-edge:

https://www.katanamart.co.uk/content/27-uk-samurai-sword-law

Samurai and other curved Swords

On April 6th 2008 a law came into effect banning samurai and other curved swords with a blade length of 50 cm or more; there are some exceptions for registered martial artists, re-enactors and even certain genuine Japanese swords.

Here is to the law:

https://www.gov.uk/guidance/import-controls-on-offensive-weapons#importing-knives-swords-and-other-offensive-weapons-into-the-uk

An amendment to this act was passed, which came into effect on the 1st of August 2008. It allows curved and samurai swords which are handmade using traditional forging/production methods to be sold without a license.

Apparently the Japanese did use straight swords for a while.

https://en.wikipedia.org/wiki/Chokut%C5%8D

Maybe the issue here is that importers avoided their previous restrictions by selling straight swords, and they're making a new class to catch those and for God-knows-what-reason are calling it a "Ninja sword".