Importing painting I already own

Hello, I plan to bring a painting that was painted by my grandfather into the UK, after it having been stored in the US for a number of years. I'm hoping to bring it in as oversized baggage, either when I travel back to the UK, or ask my husband to bring it next time he travels. I have a few questions: - I do not have an invoice as the painting was given to me by my mother, but I can provide some online auction data for the value of other similar paintings by my grandfather. Will that be sufficient to figure out the duty required - can I use the online customs tool to pay the duty ahead of time - if my husband brings the painting next time he travels back to London, do I need to provide him with any documentation that I've asked him to bring the painting for me? Thank you for your help.

Hi T Dunn,
You can use below guidance to work out the customs value for Customs Duty, import VAT and trade statistics if you are an importer or clearing agent.
Working out the customs value of your imported goods
Please check below guidance on how to declare goods for personal use.
Bringing goods into the UK for personal use.
Thank you.