Ok if we take the viewpoint that this isn't income from a trade here are some details of the chattels rules
https://www.jhpca.co.uk/tools-and-guides/your-money/savings-and-investments/capital-gains-tax-on-disposal-of-chattels
It all gets a bit complicated though if we throw into the mix the fact there was the case where the £9 million 200 year painting was exempt from tax as it was a wasting asset with expected lifespan of less than 50 years.
https://www.rossmartin.co.uk/private-client-a-estate-planning/capital-gains-tax/1074-painting-is-plant-for-capital-gains-tax
It isn't even obvious to me whether you have effectively received the 12k as disposal proceeds of the painting - or it could be argued that it some sort of compensatory payment that is not deemed to be funds for disposal of paintings.
The good news is that if its not trade related the amount is pretty much at the capital gains exempt amount - so there is unlikely to be an issue on that front unless you have other disposals.
Compensation payments - well that would probably need another whole thread to look at but i would presume - as non trade related it would either be exempt from tax or come within the capital gains exemption at worst.
Note with regard to what it actually is i would place alot more faith in the comments of pawncob and bd6759 - than anything i would add
hmmmm i am sure there is an interesting case with regard to the difference between a gun and a clock, there is a subtle difference between the two would you believe from a tax point of view! - sometimes tax just doesn't make any sense.
ahh here it is
The paragraph then goes on to consider and compare antique clocks and shotguns.
‘In our view there has to be a subtle difference between clocks and shotguns. Once you have wound up a clock, it continues to tick more than once, whereas with a shotgun once you have pulled the trigger, you only get one discharge out of the barrel. That said, we accept that you then have to go on and consider what happens if you have an automatic weapon or machine gun which effectively fires continuously.
‘While we take the view that the matter is not free from doubt, we would generally accept the argument that all types of gun should be treated together under the general description of “machinery” so that they would have a predictable life of less than fifty years,