It is certainly possible to contribute more than £20k to an ISA. You are not meant to and you get hit with a tax charge, but there is nothing that physically limits you (Freetrade could do this however).
If I contributed to a Freetrade ISA with £20,036 lump sum, and by the end of the tax year the £36 had been taken as fees, I would have contributed £20,000 to my isa that tax year. From HMRC perspective that is fair and wouldn’t be a tax charge. I assume my £36 will have to sit as cash within the account or in a sub account.
I don’t know if Freetrade will actually do/allow this, but it is possible.