KA MAN MAK

Hi, I am living in UK (work from home) with full-employ working the HK company since arrival about 3 years. The HK company has no business in UK and they pay me at my HK account and I need to transfer to my UK account. I have using self-assessment to report the tax. I have NI number. I would like to know do I necessary to pay the compulsory NI? Is it illegal? If I need, how to pay? I cannot find the location to report and pay NI. Thanks.