A study suggests that most "international experts" (whoever they are) think that people should be prevented from having IVF if they have lifestyle factors that damage their chances of conceiving, like smoking or obesity.
Since IVF is covered by NHS, but more than 70% IVF doctors work privately, it's hard to know what to do with this information. On the one hand, if taxpayer dollars are spent on a non-critical procedure for people whose lifestyles are preventing them from conceiving, it is completely reasonable to refuse service unless they stop smoking or eliminate other negative behaviours.
On the other hand, if private individuals are willing to spend money to conceive, even with a lower success rate, they should be able to do so- it's their money. They should of course be aware that it is less likely to be successful, and of any risks to themselves that they might incur. But if the only relevant difference between obese smokers and healthy non-smokers is the success rate of treatment, then the people who are spending the money on treatment should be the ones to decide whether it's worth it. If it doesn't work, who cares? It's their money.