Use the Points & Miles to Pounds Calculator to get an estimate value of how much your points or miles are worth. Select the program, enter total points or miles, and the calculator will output the average value in pounds.
If you want to get more in-depth details on the value of rewards as well as the value for other redemptions, you can click on the More Info button when applicable.
Do note that these values are based on how much The Point Calculator values these rewards. It's easily possible that you could value any of these rewards higher or lower than the listed value.
Also note that while I do cover hundreds of programs across several countries, it's a chance that the program you're looking for may not be in the program list.