Any tax due will be taken at source when the interest is paid. If your Savings Account is paying interest at 5% gross, less tax at 20% or 40% at your highest rate, although only 20% (I think) will be deducted at source, the net received will still be higher than putting all your funds in an ISA paying only 1% tax free. Under the ISA rules, you can only put a maximum of £5100 into an ISA in the current tax year and cannot keep switching the funds backwards and forwards. If you were to switch now from Savings to your ISA, any interest accrued will be calculated net of tax. The government don't take the tax at a certain date, the savings institution deduct the tax whenever they credit you with the interest. Therefore any interest accrued to date is calculated net of tax.