That doesn't make any sense.
Let me explain with simple figures not based on current rates. Say someone earns 30K, employer pays 2K in employer NI, employee pays 2K employee NI, and 4K in income tax. Summarising, the employer pays out 32K, the government gets 8K, and the employee takes home 24K.
In the new system, the employee gets 8K UBI, and the employer still pays out 32K. Their salary is 30K, but because the 2K employer-NI is now treated as a payment of their personal tax bill, it is a taxable benefit, in the same way that anything else that employer pays for on behalf of the individual is a taxable benefit. The salary is 30K, but their taxable income is 32K. In order to leave them with the same total income they had under the old system, they would have to have a total tax bill of 16K, 50% of their 32K taxable income. Since 2K has already been paid by the employer, they would actually have 14K deducted from their salary, giving them 30-14 = 16K. Add the 8K UBI and they are back to 24K.
The point of all this is just an administrative mechanism to have a flat rate payroll tax without the government losing the money it currently gets in employers NI, giving the employers a windfall reduction in wage costs. The alternative would be just to pass a law giving everyone a pay increase equal to the employers NI at the time it was abolished, but I'm assuming (perhaps wrongly) that that would be more complicated, from a legal point of view.