If you cancel your Standard or Plus plan membership and downgrade to Basic, you can still sell or hold any mutual funds or gilts that you already have. However, you’ll be unable to purchase any more without upgrading.
What about SIPPs?
If you downgrade from the Plus plan to the Standard or Basic plan, you will no longer have access to the SIPP account. If you currently have one open, you will either need to transfer the assets out to another provider or, - in the instance you're within the 30-day SIPP cancellation period, - you will need to withdraw any cash and close this account).