Hi Tom,
Until the UK officially leaves the EU, the current social security agreement with the EU continues to apply. Switzerland and the UK have entered into a citizens' rights agreement which ensures the maintenance of current citizens' rights.
The exact rules governing withdrawals of OASI benefits for UK citizens leaving Switzerland have not yet been specifically addressed. However, an extension of the current social security agreement between Switzerland and the UK would mean that UK citizens will not be able to withdraw Swiss OASI benefits upon leaving Switzerland (as is the case with citizens of EU member countries).
Best regards from Moneyguru