Don't go near an EBT.
I suggest that you post your question on
www.contractoruk.com as well.
One possibility would be to establish a UK ltd co. Costs about £50. Your uk ltd co, invoices the Swiss company for the work you've done. Your uk ltd co pays you a very small salary (~£6K), along with the employers and employees NI contributions and PAYE, and pays the rest as dividends, which don't attract NI, and, unlike Switzerland, come with tax already paid. (If you stay on basic rate).