Don't you have a contract of employment of any sort? That should specify how much notice to give. Usually you have to work the time required, but it you are owed any leave those days come on top of the notice period or you get paid for them - it all depends on your employment terms. I would say that if you are on a week's notice and you hand your notice in tomorrow, you would leave a week tomorrow, 28th July, but only your employer can confirm for sure.