Oh, that's easy to answer!
It will start as coal mined in Poland, using a lot of diesel and electric plant, conveyed to port using a lot of diesel fuel, loaded into ships by diesel hoists, then carried by sea buring a lot of particularly crude/ dirty diesel, unloaded in the UK by electric hoists, carried across the UK by diesel locomotives, and dragged up into the boiler houses by electric motor. Meanwhile, the ships will return empty/ ballasted burning more of that dirty diesel.
But none of this ever stops people thinking that somehow 'electric is better'. The losses in electric power distribution alone are staggering.
Not that I have any objection to railway electrification; I just don't see that it brings much of the benefits that are popularly claimed. It will provide electricity suppliers with a high-useage committed customer, will provide electricity generators with choice over their raw fuel which their customers don't enjoy, and will tie rail operators into a single supply, both in terms of price and availability.
It will have doubtful impact on pollutants and efficiency when viewed globally.