The reason for the around, is it's debated, there are numbers that are lower and higher than that. As said a non-coincidental point on this, a fairly sizeable number of people with severe asthma are under 16. The usual quoted figure for severe asthma is usually slightly higher than that, but it's a not a large proportion of group 4.
One of the issues here is making a comparison with the flu jab. It's a very bad idea for politicians to make a link between flu vaccine eligibility and covid-19. There is a lot of overlap, but it's going to cause the government way more issues than it's worth doing. Vulnerable is also a parlance and political word, so mix up clinically vurnerable, clinically extremely vulnerable, vulnerable and so on in political and medical landscapes and you're in big trouble.