As of May 1st 2021 the Ready to Burn legislation became an active Law. Under this new law the most polluting fuels (wet wood and house coal) have been banned.
All smaller volumes of wood (less than 2 cubic metres) will have to be certified as 'Ready To Burn', meaning it has a moisture content of below 20%.
House coal is now banned for domestic sales for use in the home.
The 'Ready to Burn' legislation also affects solid fuels. Manufactured solid fuels will also need to meet certain standards and carry a 'Ready to Burn' certification.
For Wood, look for this mark:
For manufactured solid fuel, look for this mark:
The honest answer is not a lot.
Industry standard advice has been ahead of these changes for several years. Anyone that has purchased a stove in recent years will have been advised that burning wet wood, which has a moisture content above 20% and house coal will invalidate any guarantees for a woodburner, multi fuel stove and any pipework required for the installation. More often than not operating instructions provided with stoves will clearly state that only seasoned or kiln dried logs with a moisture content of below 20% or smokeless fuels can be burnt inside the units, any other fuel will invalidate guarantees.
The reasoning behind this is not just due to emissions but also because traditional house coal and wet wood produce a great deal of soot, tar, creosote and other harmful substances that can cause damage to stoves and pipework, as well as heightening the risk of a chimney fire.
Most reputable retailers avoid selling wet wood to the general public already and sell either seasoned or kiln dried logs. Most will advise if you want true ready to burn logs to purchase kiln dried as the process they go through to dry them often gets a lower moisture content that logs that have been left to season naturally for eighteen months plus.
All reputable coal merchants and HETAS engineers who are qualified installers of multi fuel stoves will advise not to burn traditional house coal in stoves, and heavily stress that only smokeless fuels can be burnt.