Well you can just compute the value you're getting by solving a simple equation:

The formula will be:

(Cash ticket price in GBP) - (Number of Avios generated by cash ticket) x V = (Number of Avios needed for award travel) x V + (Taxes, fees and carriers charges on award ticket in GBP)

If you solve this for V, you get the value of paying that flight via Avios. I agree with the poster about the value you should aim for (V should be larger than GBP 0.01, frequently something like GBP 0.02 is achievable).

EDIT: Of course, I can solve that for you...

V = [(Cash ticket price in GBP) - (Taxes, fees and carriers charges on award ticket in GBP)] / [(Number of Avios needed for award travel) + (Number of Avios generated by cash ticket)]