Is there any reliable data on what % of the UK population currently earn less than £17,300 annually?
https://www.gov.uk/government/statistics/percentile-points-from-1-to-99-for-total-income-before-and-after-tax/

The ONS has freely available data on the percentile points of the income distribution as estimated from the Survey of Personal Incomes. The data is collected on a yearly basis, and is currently designated as "Percentile points from 1 to 99 for total income before and after tax".  

You should be able to calculate the answer to your question approximately with this information and some linear interpolation. If you really want the exact number, you probably need the actual micro-data, which I'm guessing you will need to submit a request for, unless the ONS anonymized it. 

You should check whether these still need to be deflated to get real income.