lou it really depends surely? If you are self-employed and therefore not claiming any maternity rights as such, then you are just thinking about what notice you will need to give to your clients that you will not be available, which will depend on the agreement you have with them.
It doesn't sound as though you are genuinely self-employed, you yourself define yourself as a 'worker' which sounds correct, if you have a 'boss' rather than 'clients', and presumably you couldn't decide to send someone else to do the work, provide your own equipment, work for more than one company, make your own decision about what and how much work to do, risk your own money, etc? If you have boss who gives you work you wouldn't be classed as self-employe regardless of who pays your tax.
In which case as you will see from the link, you do have entitlements to pay and other protection, and should give the same notice as an employee.
Either way, it would be courteous to give as much notice as you can to enable your boss to make plans for alternative arrangements, particularly if you are hoping to work for them again in the future.