The actual import cost - or customs duty - is 3% EU-wide for guitar-style string instruments, as far as I know. The issue comes with VAT which you now have to pay for goods bought from the UK (unless the seller does that for you) and is dependent on where you live. So since Danish VAT is 25%, the total added cost would actually be 25% + 3% = 28%. For me, living in Finland, it would be 27% since our VAT is "only" 24%. Nevertheless, all of this makes buying goods from non-VAT registered shops or individuals in the UK a total no-go, at least if you're buying anything expensive like the uke sold in this thread. Thankfully, shops like SUS let you buy goods with the UK VAT already removed from the price which increases the prices only a relatively small amount, and some shops like World of Ukes will pay the VAT for you so their prices remain the same as pre-Brexit. But, as I said, buying from individuals unfortunately became much more expensive post-Brexit.
Good luck with the sale, Counter. That's a brilliant looking uke but not an easy sale, it seems.