The Trading 212 ISA is not flexible and if you withdraw funds and deposit them back, they will count for the current tax year once again. For example, if you have deposited £5,000 and decide to withdraw £500 the balance in your ISA might be £4,500, but your current tax year subscriptions will remain £5,000, and any deposited funds later will be added on top of these subscriptions.
Note: This is also valid for funds that are transferred between the ISA and another type of account (CFD or Invest).