You can only open an IBUKL Stocks and Shares ISA under a GBP cash account type. A cash account requires that all transactions are made with available cash or long positions.
Additional Information
Under what IBUKL account type can I open my IBUKL Stocks and Shares ISA?
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Can I cancel my IBUKL Stocks and Shares ISA?
Can I trade on margin under the IBUKL Stocks and Shares ISA?
Can I link my ISA to an Introducing Broker?
Can I trade on margin under the IBUKL Junior Stocks and Shares ISA?