The changes to tax relief for travel and subsistence only affect those who work through an employment intermediary. The planned changes will put supply teachers employed through an intermediary on the same terms as other supply teachers, either contracted directly, or through an agency contract.
The Government’s general assessment of the effects of the measure can be found in the Tax Information and Impact Note: https://www.gov.uk/government/publications/income-tax-employment-intermediaries-and-relief-for-travel-and-subsistence/income-tax-employment-intermediaries-and-relief-for-travel-and-subsistence
The Government undertook detailed consultation on these proposals. Further assessment can be found in the summary of responses to the consultation document published on this change: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/483389/Employment_Intermediaries_and_Tax_Relief_for_Travel_and_Subsistence_-_Summary_of_Responses__M7057_.pdf