This question was answered on 11th January 2019
The estimated number of taxpayers liable for tax at the higher rate are published in the HMRC National Statistics table 2.1, available here:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/710887/Table_2.1.pdf These estimates are based on the Survey of Personal Incomes (SPI) outturn data up to 2015-16. The 2016-17, 2017-18 and 2018-19 estimates are based upon the 2015-16 Survey of Personal Incomes projected using economic assumptions consistent with the OBR’s March 2018 economic and fiscal outlook.