Don’t get cute with your emergency fund. Keep it in a bank account where you can get instant access to it if you need it. Some will say you should keep it in something like Premium Bonds, because you could potentially earn more interest.
You could, but it could also take weeks to get your money out if NS&I drag their feet with the withdrawal or they're overwhelmed by requests. This happened at the beginning of the like they were the beginning of the pandemic.
The point of an emergency fund is not to earn interest. The point is to have instant access to cash in the event of an emergency. Make sure you’re using the right tool for the job.