Actually I would say NO you can't earn extra money whilst on materntiy leave.
SMP is there because you are not able to do your normal job because you've had a baby. During this time you can "work" 10 days for your employer as keeping in touch days.
any more than this then you do not receive SMP for those weeks. So be very very careful as you could end up loosing out..
There is a good fact sheet on the maternity action website that explains the keeping in touch days better than me. http://www.maternityaction.org.uk/workingparents.html