You can't take annual leave and receive SSP at the same time, but you can take annual leave for some of the time you have left, yes, as long as your employer agrees to the holiday request. They don't have to - as long as you don't lose the holiday. I would expect them to let you take it though.
You can do this, as long as your employer agrees! Just also bear in mind that if you’re off sick for a pregnancy related reason in the last four weeks before you’re due, your maternity leave will automatically start early (if it hasn’t started already by that point).
As pp said if you get signed off sick up to your maternity leave, they will start your maternity leave early. If you have any accrued holiday that you have not been able to take, it is usually added to the end of your maternity leave. If they business don’t want to do that then they might pay it. If you have an Hr dept, check the policy with them. If not, give Acas a call.