And you have to tell HMRC that you have allocated personal allowance in that way in the "white space", otherwise they will calculate incorrectly. No, they will calculate correctly, based on the information you have provided. If you have not given the additional information in the "white space" how can you expect them to take it into account? For most people on self-assessment HMRC calculate the tax due based on the figures provided by the customer. HMRC are not omniscient, they can't be expected to understand your intentions if you don't provide the information.
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