As long as you are meet the SMP conditions (employed for 26 weeks by the 25th week of pregnancy -i.e. were pregnant before you started working there; have average earnings above the lower earnings limit - £107 per week in 2012/13 and £109 in 2013/14) then it makes no difference what your status at work is.
The only difference being signed off sick would make, is possibly reduce your average earnings if your sick pay is less than your normal pay, and your sick pay takes place during weeks 17-25 of pregnancy, because those are the typical 8 weeks that your pay is averaged over to define your average earnings.
You have to be employed up to the 26th week I think to get SMP. You might still be entitled to Maternity Allowance (which is the same amount) if you have been employed for 26 weeks out of the last 66 weeks.
AFAIK, the only time being signed off with a pregnancy related illness matters is if it is after 36 weeks - your employer can make you start your ML early.
It is illegal for them to sack you because you are pregnant of course.