The purpose of the tax is not to raise money to plant trees, it’s to raise the cost of emissions so that markets move away from them
The parent comment stated "actual cost has to price in the impact of using it". Most people would agree on this. However, for both claims to be true, the collected tax revenue must be spent offsetting the impact of that gas usage - not simply reducing gas usage (ie. that consumed gas isn't being compensated for).
If the UK government is spending that tax revenue on anything it wants, then it's not the actual cost, is it?
If you then also spend the taxes on some form of offsets (if we pretend for the sake of argument that those work) you would have reduced emissions twice. One time seems plenty to say they are doing their job.
Is there real evidence the collected tax revenue is actually offsetting carbon emissions?
There's a lot of fraud in carbon credit systems - where often the sole benefit is feeling and/or looking good.
Is this self-imposed tax actually having a real result - or is it just artificially increasing the price of energy? If the latter, then it's not really fair to claim it's the actual cost.