Hi,
In the "address_vat" custom field, you can use the "display limited to" setting to restrict it to the "address_country" field with the value "United Kingdom" and it should appear only in that case. And if you set the "address_vat" field as required, it will only be required when displayed by the "display limited to" setting.
Regarding the check, it's done after you click on next.
Note however that since brexit, UK VAT numbers check has been removed, so it won't do the check.
So if you want to do a check, the only thing left is to use the "regular expression check" setting to enter a regex for the format of UK VAT numbers:
(GB)?([0-9]{9}([0-9]{3})?|[A-Z]{2}[0-9]{3})
as per
www.oreilly.com/library/view/regular-exp...9327453/ch04s21.html
Finally, I'm not familiar with the rules for VAT charges in the UK since brexit so I wouldn't be able to recommend what to do. I would recommend you check first with your UK accountant, or your UK local tax office what they recommend. Then, based on that I can tell you how to setup the tax rules in HikaShop based on what you should and what you should not charge.