I suppose it would depend where you are resident . If you are a UK resident, as defined in the RDR3 doc
https://www.gov.uk/government/public...dence-test-srt
then you would have to declare your worldwide income to HMRC and pay any difference. I can only assume that historically people may well have forgotten to declare, however, since Jan 2016, China and the UK/EU have automatic information sharing and I would suggest not declaring it to HMRC could lead to a world of hurt in the future.
Same would go for those resident in the EU, unless you were on a scheme something along the lines as Portugal’s NHR scheme.