Any work that you do on a self employed basis during your maternity pay period will not affect your entitlement to statutory maternity pay (SMP), so your current employer should still pay you this.
It is important, though, to first check your contract of employment to see if there is a clause in there that prevents you from taking up any self-employed work whilst employed by your employer. Whilst on maternity leave, you are still employed by your company so are therefore still bound by the contractual terms.
Even if you do not intend to go back to work with your employer after your maternity leave, you would still be entitled to receive SMP so long as you were still in their employment in the ‘qualifying week’ which is 15 weeks before the week in which your baby is due.
The situation would be different if you actually began work for another employer who did not employ you in the qualifying week, in which case you would not be entitled to receive SMP from your original employer from the point you begin work with the second employer.