Full time at statuatory age?

Bit confused about this and not sure where we stand...

My daughter (4yrs) is currently at mainstream school part time and isn't of statuatory age until next term so can be part time. However, when next term comes along does she HAVE to be full time? Here's why I ask...

She was at pre school 6 hours a day last year, last term at school she was there 4 hours a day and now, this week, they have just reduced it further to 2 hours a day because the school say she isn't learning anything or benefiting from being there for any longer each day.

We are awaiting ASD diagnosis/assessment from our CDC and she also has epilepsy which is not yet fully controlled.

TBH she is bored out of her head at home and to me, her being at school so little isn't helping her social skills (or learning - but they are teaching her at 22-36 month level which is where they feel she is at).

I just wonder what will happen in April when she becomes statuatory age - can they keep her part time still? We are looking into SN schooling at the moment, so would it be the case that she has to be in FT education somewhere and if the mainstream can't accomodate her then she will move to SN school ft? I haven't got a problem with this idea atall, I am just trying to find out how it works....! At this rate my 2 year old toddler is gonna be at 'school' more than her by next term!!

Thanks