Voluntary Carbon offsetting can be achieved by giving the funds to UK PLC Charity, who then make donations to carbon offset projects.
Because offsetting is voluntary, your offset is treated as a donation to the charity, and any tax reclaimed on your donation will be used by the charity for its general charitable purposes.
The main use of any tax reclaimed, will be on helping other charities become more both efficient and carbon neutral, since the internal operations of most charities are the very last area to receive any funding and this does make charities both very vulnerable to administrative and other process errors and often reduces their effectiveness.
By making donations to charities that improve the way they operate UK PLC Charity is often able to increase the effect of a donation by a factor of 5-10x vs a straight donation to the receiving charity.