Carrying a kubotan in public is where you risk serious legal trouble:

If stopped by police, having an item designed primarily for self-defense can lead to arrest. The burden of proof often lies with the individual to provide a "good reason" for carrying it that is not related to self-defense. Selling, buying and carrying knives and weapons - GOV.UK

Yes, you can generally find and buy kubotan-style keyrings from specialized retailers like Enso Martial Arts . However, the (a cylindrical container holding sharp spikes) is strictly prohibited as an offensive weapon per se under the Criminal Justice Act 1988 . Is it Legal to Carry?

In the UK, buying and carrying a kubotan for self-defense is a complex legal area where and design determine whether you are breaking the law . Is it Legal to Buy?