They dont. Manufacturers do not actually pay the tax. Instead sodas lowered the sigar level to below 6g per 100ml. From 10 to 12 before.

So full coke in the UK has no more tax then diet.

To confirm this. I just discovered coke is now selling a classic taste version. With 10.6g sugar per 100ml.

And sainsbury charges 21p more for 8 cans then diet coke.

If you look at pepsi full. It is 4.6g now rather then >10g.