For a government document this one sets out the situation for importing to the UK relatively clearly Notice 143: a guide for international post users - GOV.UK (www.gov.uk).
For someone resident in the UK receiving a concertina from abroad worth over £135. They will have to pay VAT at 20%, though if you have bought the item, many countries have schemes whereby you should not have to pay their local sales tax. There may also be import duty due. There are internationally used codes to identify items for the purposes of duty. 9205 90 10 Accordions and other similar instruments. In this case the duty is 2%, Concertinas over 100 years old can be classed as 9706 00 00 Antiques of an age exceeding 100 years and they are free of duty. One thing to watch is the VAT is not only charged on the value of the goods, but also on the duty, insurance and postage and packing. The only real change is these charges now apply to EU.
For people buying from the UK. If the seller is VAT register there is a scheme where they can sell free of VAT Exports, sending goods abroad and charging VAT - GOV.UK (www.gov.uk) The EU an USA treat antiques the same as the UK so there is no duty payable. In the EU you will have to pay the local applicable VAT.
You can look up commodity codes here The Online Trade Tariff: Look up commodity codes, import duty, VAT and controls - GOV.UK (trade-tariff.service.gov.uk) you can also find the UK rate of duty.
You can also find the commodity codes and EU rates of duty here. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=OJ:L:2020:361:FULL&from=EN Note they us a comma instead of a decimal point in the % duty rate.