I really feel for you Evaluna, this trap where you dare not earn more but don't earn enough. Is there a way to appeal if you can show you don't regularly earn that amount, or do you need to avoid overtime hours, or the hours you get paid for in one go? If you don't have to do overtime regularly, is it possible legally to spread them over a few months so you put a few on your time sheet each month not to go over the threshold? A bit like annualised contracts for people who don't work so much in School Holidays, but get their pay spread equally through the year? Maybe an email to your local Citizens Advice Beureau would give you an answer (doesn't help this month though).

Good luck with the future, and I hope no more nasty surprises for a good long while!