Is a student loan with interest considered usury, given that the loan is not repaid if the student's annual income does not exceed £21,000? And does this loan affect the blessing in one's sustenance and good deeds?

Every usurious loan is forbidden, even if the borrower believes it's possible to avoid paying interest, because simply by entering into the contract, he participates in a usurious agreement. If you proceeded with this loan based on a fatwa that permitted it, then there is no sin upon you for what has passed. You must strive to repay the debt and extricate yourself from its usurious effects as soon as you are able. And whoever repents with sincere repentance, Allah will accept his repentance.