As you can see the picture above illustrates that sustainably sourced wood fuel has a relatively low effect on CO2 level in the atmosphere. This is because for every tree that is burnt another takes it's place and by simply growing absorbs the CO2 that was released by burning the first tree - simple.
Obviously some CO2 is produced as a result of the processing and transport of the fuel.
If timber is of building quality, and would be used for building with otherwise, then it is better to use this wood for building with and other parts of the tree for burning. Timber production is a little limited in the UK for a variety of reasons some historical, some to do with our climate, and in fact woodfuel production and building timber production often go hand in hand.