ISAs are tax-advantaged savings products. No tax is payable on interest or capital gains arising on investments held within ISAs, and therefore no revenue accrues to the Exchequer as a result. In 2007-8 the tax relief provided through ISAs amounted to an estimated £2.4 billion.
A small amount of revenue may accrue to the Exchequer in the case of uninvested cash which is held for a period of time within a stocks and shares ISA, however this is not a result of tax. This is because interest earned on this cash is subject to a flat 20 per cent. charge to prevent abuse of the ISA regime. HMRC does not collect specific data on the income from this charge.