Are you working full-time (30 hrs or more per week)? If so, then you are not entitled to Supplementary Welfare Allowance (SWA) regardless of what your income is.
If you are working less than 30 hours per week, you should have been means-tested on your wages (and savings if any) to see if you were entitled to Supplementary Welfare Allowance. Were you asked for a payslip when you applied? Were you means-tested? If you weren't, then you should ask for an official refusal letter from the SWA officer and appeal the decision.