Tax will be paid at your marginal rate. Which basically means that anything over the £30k tax free limit will be classed as income and added to any other income you’ve earned in that particular tax year.
If your total income(not including the £30k) is more than £45k then you’ll pay tax at 40% on the amount that’s over that limit. Otherwise it’s 20%.https://www.gov.uk/income-tax-rates/cur ... allowances
In practice most posties won’t pay tax at the 40% rate.