If that is your 'Total' source of income, you will only pay 20% on the amount left after your basic allowance is deducted.

20% tax. 11% NI. About £11,500.

You have a yearly allowance of 11K so will only pay 20% on 4K

There is a free, official, calculator on gov.uk/income tax.

you still earn the 15K, what you take home after taxes is what you take home, NOT what you are earning

you are taxed on the 15K, that is what is reported by your employer etc

As we do not know your tax code we have no idea.

Hmm! Not enough to splurge!