Sustainable is about not diminishing the fuel source, fossil fuels are not sustainable because we can't replace them as quickly as we use them. We can with wood, especially fast growing soft woods like pine. Sustainability does not affect air quality.
Cleanliness and pollution is about how much crap goes into the atmosphere. This affects air quality.
If every London household had a wood burner and the same heat requirements that you suggest, the entire national forestry stock would be exhausted in 18 months.
You're mixing things a bit.
Sustainable is about not diminishing the fuel source, fossil fuels are not sustainable because we can't replace them as quickly as we use them. We can with wood, especially fast growing soft woods like pine. Sustainability does not affect air quality.
Cleanliness and pollution is about how much crap goes into the atmosphere. This affects air quality.