There is no absolute prohibition on owning nunchuku (rice flails) in the UK.
They are deemed to be an offensive weapon 'per se' ie: their prime purpose and reason for manufacture, is to cause damage. Therefore to possess them in a public place you must be able to demonstrate lawful authority or reasonable excuse.
Each incident is naturaly dealt with on a case by case basis. However, I would suggest that transporting them in a training bag, directly to and from your home and place of training is not a problem. Nor would be using them at your place of training.
Using them at home naturaly requires no reasonable excuse to be shown, as this (I assume) is not a public place.