Self employment is a minefield for anybody who is low paid. The sad fact is that if you become self employed, you have restricted access to certain benefits like unemployment benefit if your source of income fails ie no students or bad weather! You must also pay your own National Insurance Contributions (Self Employed Stamp) If you are working for one employer, you are employed not self employed and it is possible that sometime in the future the IR may determine that you are actually employed rather than self employed with all the complications that that will cause to both employer and you. Why is your prospective employer looking to take you on on a self employed basis? Maybe he/she want's to pay you below the minimum wage, maybe he/she doesn't want to pay you when you are sick, for holidays or Bank Holidays, maybe he/she just want's to treat you like a dog and throw you a few scraps just to keep you from starving! On the plus side, it is possible to claim for many things against income tax including your car (excluding home to place of work travel)
The advantage of self employment is to that you can chose to work for more than one employer and possibly increase your earnings potential.
If you are not able to sort out your own book-keeping, you may well expect an accountants bill of around £500 for the year. Get a quotation before employing one on a send me a bill at the end basis!
Think hard and good luck!