If you cancel your Plus plan subscription, your monthly payment will continue at £4.99 per month for the Standard plan unless you also decide to downgrade to the Basic plan.
If you do wish to close your ISA, you’ll need to empty it or transfer it. Once done, you can close it in the app: “Portfolio” > 👤 Profile icon > “Manage my ISA”
Please note that closing your ISA will not automatically downgrade you from the Standard plan, this will also need to be done in-app.