I see a sugar tax as a way of creating a fund to enable councils to implement healthy travel options (protected cycle to school networks, discounted public transport, public transport networks) and mitigate the cost of obesity-related diseases to the NHS.
Here is a study on this subject: http://www.bmj.com/content/349/bmj.g4887 Its conclusion: "Men and women who commuted to work by active and public modes of transport had significantly lower BMI and percentage body fat than their counterparts who used private transport."
http://webarchive.nationalarchives.gov.uk/20130103014432/htt...
In my view sugar has been used more and more as an extra ingredient in other foods. Additionally, there are different kinds of sugar, which may influence your statistic: http://thatsugarfilm.com/blog/2015/03/16/added-sugar-vs-natu...
I thoroughly recommend watching That Sugar Film.