Hi maria.barker,
Please excuse the delayed response.
Because Switzerland has a bilateral social security treaty with the European Union (EU), it is not possible for a UK company to hire an employee in Switzerland on an ANobAG basis. The ANobAG category only applies when employers are domiciled in countries which do not have social security agreements with Switzerland.
As an EU-based employer, the situation is more complicated. Depending on various factors, the employer and the employee may have to participate in either the Swiss or the UK social security schemes.
You can find the details of EU social security regulations here:
https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:284:0001:0042:en:PDF
https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2004:166:0001:0123:en:PDF
If the resident of Switzerland is able to maintain their Swiss self-employed status, this may be less complicated for both parties. The employer-portions of Swiss social security contributions which the self-employed person must pay themselves could, for example, be compensated by the employer by way of higher payments for services rendered.
If they are unable or unwilling to continue working on a freelance basis, consulting with a law firm that specializes in cross-border employment within the EU may be a good first step.
Best regards from Moneyguru