Best to see an accountant and make sure you get this right or it could be very expensive For both you and the company when you sell the shares in the future.
It’s complex and can be a minefield.
If the company is worth 1 million and you buy 10% then if you pay 100 K then there shouldn’t be a tax liability on you or the company as you’ve paid market value. If you pay £90k there is tax/ni on £10k (the underpayment) + as you’ve underpaid 90/100 of future gains taxed at income rates plus NI (employer and employee).
However, you need to be careful about how any restrictions in connection with your shareholding impact valuation and how much you pay should pay for the shares.
You will definitely want to enter into a section 431 election to protect you and the company against possibly penal tax charges when you come to sell the shares assuming you make a gain. The 431 election fixes the income tax charges to any “gain” when you buy the shares not when you sell.
If no one mentions s431 elections find another advisor.