HMRC have examined their data systems and because of the way the data is held, they cannot construct an average of sole trader profits over several years for such a large group within the resource constraints for a Parliamentary Question.
Eligibility for the SEISS is based on average trading profits for sole traders and income from partnerships. More information on the eligibility criteria can be found here: https://www.gov.uk/guidance/claim-a-grant-through-the-coronavirus-covid-19-self-employment-income-support-scheme.
Those with average profits above £50,000 are not eligible for SEISS but could still benefit from other support. The SEISS supplements the significant support already announced for UK businesses, including the Coronavirus Business Interruption Loan Scheme and the deferral of tax payments. More information about the full range of business support measures is available at www.businesssupport.gov.uk/coronavirus-business-support/