Above (click on picture to make it larger) shows that sustainably sourced wood has a relatively low impact on CO2 levels in the atmosphere. For every tree burnt, by simply growing another absorbs the CO2 that was released.
There is obviously some CO2 produced as a result of processing and transporting the wood.
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.
The forestry commission have set up the following: