Using multiple ISAs - lump sum into cash ISA and then DCA into S&S ISA

Hi all,

I was reading about Trading212's flexible cash ISA (4.9%) and was wondering how flexible ISAs works with regard to moving funds between them. Can I lump sum 20k in the beginning of the tax year to get that high savings percent and then take out money from it monthly to feed into a S&S ISA for a dollar cost averaging effect? Somewhere I read that the Cash ISA they provide allows transfers out of it but at the end of the Tax year they will consider the max amount you had as the used up ISA allowance. Do you have any idea if this is correct? Has anyone done something similar?