Exactly. This article doesn't even rank Birkenhead:
The top five overweight cities and towns in the UK included Boston - with 34 per cent obese, Barnsley, Mansfield, Rotherham and Fenland.
This article, from September, shows Wirral isn't too bad:http://www.telegraph.co.uk/health-fitness/0/englands-obesity-hotspots-does-area-compare/
As for the original article, besides being from 2016 and based on old data, the data is actually:
Out of every 100,000 people in the area there were 2,894 admissions where medics recorded a "primary or secondary" diagnosis of obesity.
This doesn't mean that Wirral is the fattest region, it just means that more cases have been recorded by doctors in hospitals in the area.
This could be down to more observant doctors, more doctors just putting down "obesity" as a matter of course, or more people being referred to Wirral.