ok i know this isn't going to be very popular, but i support their decision.
It's right to say that smoking and drinking etc affect health too - and that's part of the assessment process. Many councils now will not allow smokers to adopt at all, some won't allow them to adopt if they smoke in the house where the child is going to be living, and some even do tests to check if you are telling the truth! Same applies to drinking habits. And certainly to drugs.
The reality is that some of this is a numbers game. People who have a bmi of over 40 (which in the case of this council is the cut off) are much more likely to die early, get diabetes, be ill and be unable to support a child having a healthy lifestyle. Of course there are always exceptions, and many people with a bmi of 45 might be happy and healthy, but if someone is far more likely to be ill then this must be taken into account.
On the issue of naturally creating children: That's not a fair comparison. Councils are working with children who have already been rejected by their natural families. There are very few babies up for adoption nowadays. The vast majority of children are chaotic and traumatised and need excellent parents. The council's job is the find the very best parents possible, not to draw a line and say 'well if you can have children naturally then you can adopt'.
It's highly unlikely that someone with a high bmi would have their children taken off them simply because they were fat. But when a council makes a decision to place a traumatised child in the care of parents, it tries to find the very best parents and does so taking into account a large number of factors, including the health of the potential adopters.
Personally I think there should be some kind of 'responsibility' checks done before anyone has any children by any means.