@Manna I think you may not have yet understood. The ISA regulations do not allow shares to be moved from a GIA to ISA account in the way you are wishing to do.
It is required that the shares be sold in the GIA first and any capital gains tax paid. Only cash can be moved into an ISA account. This is not something that Trading 212 has any power to change.
Some brokers offer something called “bed and ISA” whereby they complete the sale and repurchase for the cost of a single commission rather than two. But as Trading 212 has no commission fee this is moot on their platform. Trading 212 also makes your funds from a sale immediately available to you for moving into the ISA, whereas on some platforms you would have to wait two days for settlement.