So, jobs A & B ARE paying the same but because job A has my tax free allowance then they're paying less tax then job B?
Yes. Income tax is calculated on your total earnings across all jobs.
You are paid a Gross salary in each job, the employer pays Employers NI on top of that Gross salary. Income Tax and Employee National Insurance are deducted from the Gross, leaving your Net Pay, which you actually get in your bank account.
Would it help to give an example of what happens if you just had one job, paying £12 gross an hour, for 41 hours per week?
£12 x 41 = £492 (weekly gross salary)
£492 x 52 = £25,584 (annual gross salary)
Income Tax:
0-11,000 = 0%
11,001-25584 = 20%
If you earned 32,001 or above, then there would also be higher rate tax.
So first £11,000 = 0 Income Tax.
11001 to 25584 (so £14,584) at 20% income tax = £2916.80
With two jobs, one at 20hours and 1100L tax code and the other 21hours and BR tax code, both £12 gross per hour:
Job A:
20 x 12 = 240
240 x 52 = 12,480
Minus £11,000 personal tax allowance = £1480
20% tax on 1480 = £296
Job B:
21 x 12 = 252
252 x 52 = £13,104
No tax allowance, so all taxed at 20% = £2620.80
So income tax total: £296 + £2620.80 = £2916.80
Which is exactly the same as the earlier figure when having just one job.
Do you see how Income Tax is calculated?