If you remove a mature tree and replace it with a young one, you will actually increase uptake as the growing tree will absorb more CO2 than a mature one will.
However, I agree that it is complex because you need to take a long term view and there are always risks. For example a wild fire would offsetting the equation as the young trees are more vulnerable.
You said it, very lazy
I'm not familiar with how the UK decides on dispatching order for power plants, but if they follow a similar protocol as the US where is a combination of marginal cost and emissions, I wound in then expect that the bio-mass plant (with lower expected emissions) will be dispatched more often than the coal fire power station.
That would significantly affect the emissions/kWh
Finally, like you said we would need the transportation emissions and I would ask too for info on whether the source of the wood is a sustainable managed forest. If it is, that wood has near zero emissions as the forest regrows (except for processing emissions)