Yes give your normal notice period, but wait until 11 months, not 3. You'll continue to get SMP anyway but if you stay employed throughout your full year maternity leave you'll also get holiday pay and any other contractual benefits, so make sure you don't leave before your maternity leave would be up.
Unless you're going to start work elsewhere that is.
Hello, I'm thinking about not going back to my job after maternity leave. How would I do this? Basically I'm only getting smp so no need to worry about paying anything back. Would I, for instance, say I wanted 4 months maternity leave but then as I approach the end of month 3 "hand in my notice" for the 4th month is my last? Hope this makes sense.