This week the best interest rate for instant access savings account is from ING Direct with 2.6%, 20% tax = 2.08%, 40% tax rate = 1.56%

more information: ING Direct savings accounts

* New customers can enjoy a guaranteed rate of 2.6% AER for 12 months
(2.57% gross p.a.). Interest paid monthly.
After 12 months the rate will revert to our ING Direct Savings Account variable rate, currently 0.50% AER (0.50% gross p.a)
No charges for withdrawing your money
Start saving from £1, with no minimum or maximum monthly deposits
Save up to a total balance of £1 million in your ING Direct Savings Accounts