Why does Britain not have a system of Relative taxation?

Because it would be stupid. We have a progressive income tax and there is a good case for a local income tax to replace tax based on property value. You could argue for them to be more progressive but unfortunately there is a point when higher marginal taxation on the wealthy leads to reductions in tax revenue. Instead we need to genuinely look at reducing wasteful government spending and focus any savings on reducing direct taxation on low paid workers.