It takes 3 to 5 years for most businesses to start turning a profit for the government to even tax.

The owner will have been paying themselves a salary to live on during that time. Once they become profitable, assuming they want to be "tax efficient", they'll still pay themselves a salary because that £12,500 tax free personal allowance is just sitting there otherwise. They have another £2000 tax free of dividends. Under Labour, they will have to pay 20% on what's left over instead of 7%. But since the tax rate is just the same as income tax now they might as well just make the whole thing a salary, which the business is now no longer taxed on. Plus, I think there's other stuff they can do to be "tax efficient" - IDGAF, this policy isn't aimed at them.

This is not going to stop anyone started a business, it means they'll take home a bit less several years down the road. It means they will, in fact, pay the same tax rate their own employees do.