IR35 is off payroll working and would apply to a worker, who provides services to a client through their own intermediary and who would otherwise be an employee, if it were not for the intermediary. Under IR 35 rules, tax and National Insurance would be deducted. You can see guidance here:
Understanding off-payroll working (IR35)
SA103F, box 82 is where you would declare 'other tax taken off trading income', if the tax deducted is not part of the Consruction Industry Scheme.
If you have regisered for Self Assessment as self employed, class 2 and class 4 National Insurance will be calculated by Self Assessment.