maternity pay is calculated on the same period for everyone and it's then averaged. the weeks taken into consideration are somewhere between weeks 20 and 30 of pregnancy. therefore, you will still be on pay protection, so it will be this that is taken into consideration.
someone will be along no doubt soon with the correct weeks. But it can be in your favour to work all the unsocialable hours you can during this period (if you receive unsocial payments); book additional oncall or take on overtime as this will increase your average earnings. obviously, only do this if you are taking nhs enhanced package.
SMP is calculated based on your earnings between weeks 17 and 25 of your pregnancy. Your maternity policy should tell you whether the NHS enhanced pay is calculated in the same way or is subject to change if the individual's salary goes down.