If you have not been given a contract or written statement the period of notice you need to give is probably no more than the statutory minimum requirement for employees and workers.
There is nothing to stop you saying you’re leaving immediately and not going back at all, but they may owe you money and this may damage any reference you want from them.
The notice period is there so that your employer can find someone to replace you.
When you are ready to resign you just need to write them a short letter saying you are leaving and when your last day will be.
It would be a good idea to ask for a written statement or contract in the meantime as you are legally entitled to be given this within 2 months of starting in a job. It may be that your contract/statement will ask that you give them more than 1 weeks notice, but you haven’t been made aware of this (your Employer can specify a longer notice period in your contract, that is more than the statutory minimum).
Photo by zoomar