Before you start the process, it is important to make sure you have no money going to investments.
Go to Brain in the app and tap Splitter. Make sure the splitter is set to only go into pocket(s).
To transfer ISA out of Plum you would need to request a transfer form from your new provider.
On the form, they will request the investment account number. You can find this in any trade confirmation and Statement documents. To find these go to settings (in iOS the icon on the top left or in Android tap 'You') and select Documents. It is the long set of characters that is a mix of numbers and letters, formatted like xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.
They will also ask who the provider of the account is. The product provider for the Plum ISA and GIA is a Plum group company, Saveable Limited (authorised and regulated by the FCA).