I have a few similar to these, LAIX Pens, I see nothing to stop you using them as a pen. The point isn't concealed and it's not a kubaton. Amazon sells them and they are very hot on weapons. I regularly use them in my bank writing paying in slips and cheques, never a raised eyebrow.

What counts as an offensive weapon

Offensive weapons are goods designed to kill or inflict serious injury which have no real legitimate use. Such weapons are restricted and in the case of flick and gravity knives are banned from being imported into the UK and can be seized by UKBA officers. These include:

knuckledusters, handclaws and push daggers
footclaws - ie spikes designed to be strapped to the foot
flick or gravity knives - ie with blades that are spring-loaded or can be opened using gravity or a flick of the wrist
weapons with a concealed or disguised blade or sharp point - eg swordsticks, stealth knives, butterfly knives and belt buckle blades
martial arts weapons such as death stars, hollow kubotans and kusaris
batons and telescopic truncheons
blowpipes or blowguns, except for use by vets or registered animal handlers
curved blade swords with a blade over 50 centimetres