Interactive Investor is a UK flat-fee investment service with more than 400,000 users with over £55 billion in assets under management. It offers a vast selection of investment options, including trading, SIPP, and ISAs. The Stocks and Shares ISA starts at £9.99 per month (“Investor service plan”).Interactive Investor is a UK flat-fee investment service with more than 400,000 users with over £55 billion in assets under management. It offers a vast selection of investment options, including trading, SIPP, and ISAs. The Stocks and Shares ISA starts at £9.99 per month (“Investor service plan”).