paying tax

So at Tesco you're getting 230 x 4 = £920 a month, roughly £11,040 a year

At Sainsburys you'd get 294.40 x 4 = £1,177.60, roughly £14,131.12 a year

You can earn up to £11,850 this tax year without being taxed, you'd get taxed at Sainsburys but not if working at Tesco as you don't earn enough

The basic tax rate is 20% for the amount you'd be earning, so you'd lose 20% of your earnings and your net pay at Sainsburys would be £11,304.96

So even though at Sainsburys you'd be taxed, you'd still be roughly £300 better off

Not quite. The tax rate applies to your earnings above the £11k threshold, so the actual amount of tax OP would pay is less than you've calculated. Their actual take-home pay (accounting for both tax and NI contribs) for the year would be £12,992.