Under current UK tax rules, the effective rate of import tax on the import of the work of art from the US to the UK is usually 5%. Once it has been imported into the UK, you would be free under current EU rules to transport the work to say, Italy, without incurring any further import-related tax charges. If you then brought the artwork back to the UK under current rules there would also be no further tax charge. However, these rules may change from 31 December 2020.
Unfortunately the detailed structure of the UK import tax regime as from 1 January 2021 currently remains unclear. Hopefully the new UK legislation will be in substantially the same form, however, in order to be sure of the tax position, it is advisable to import the work of art to the UK from the US before the 31 December 2020 if that is where you intend to keep it for the long term.