I’d also presume that the consumption of sugar replacements has increased by an amount related that of sugar’s decline. In the UK you’re hard pressed to find any actual real sugar in soft drinks since the tax was imposed. Instead most things are packed to the gills with alternatives.

What do you mean by "actual real sugar"? Do you mean a specific chemical like fructose or sucrose, the source of the sugar, or something else?

Fanta, Sprite, Dr Pepper and Pepsi all reduced their sugar content to around half (~5g/100ml) following the sugar tax. Coca Cola still has it up at ~10g/100ml. They all still have sugar in their nutrional info. I'm not sure that the source of the sugar has changed, but I don't know what the source actually is. I know that in the US they tend to use high-fructose corn syrup because it's cheap, but that the EU restricts import of this. Would be interesting to see whether any changes have happened here as a consequence of Brexit.