More accurately one can write down the exact value of £1 paid at some future date as
|£1 x ( 1 / (1 + R) )|
where R is the interest rate for the period (assuming this is less than 1 year). A similar equation applies for periods of greater than one year.
The amount 1 / (1 + R) is known as the Discount Factor and is used to find the value in today's money of a sum of money paid at some time in the future. This value is sometimes called the "discounted" or "present" value.