I assume you have already travelled London - Zurich, and it is your return which has been cancelled, stranding you in Switzerland.
Whilst Switzerland is not in the EU, it is a signatory to EC261/2004. Thus cancellation of a departure from Zurich is covered by that regulation.
Since these are extraordinary circumstances, you will be unlikely to receive the usual levels of compensation for a cancellation within 14 days of scheduled departure.
However, this is distinct from the duty of care provisions. Provided you have not opted for refund on your cancelled flight, then guidance issued just yesterday confirms that this still applies, despite (or arguably, because of) a potentially lengthy stay:
According to Article 9 of the Regulation, which provides all relevant details, passengers
who are affected by a flight cancellation must also be offered care by the operating air
carrier, free of charge. This consists of meals and refreshments in a reasonable relation to
the waiting time; hotel accommodation if necessary, and transport to the place of
accommodation. Moreover, airports are to provide assistance to disabled passengers and
passengers with reduced mobility in accordance with Regulation 1107/20069
.
It is worth recalling that when the passenger opts for reimbursement of the full cost of the
ticket, the right to care ends. The same happens when the passenger chooses re-routing at
a later date at the passenger's convenience (Article 5(1)(b) in conjunction with Article
8(1)(c)).
The right to care subsists only as long as passengers have to wait for a rerouting at the
earliest convenience (Article 5(1)(b) in conjunction with Article 8(1)(b)).
The intention underlying the Regulation is that the needs of passengers waiting for their
return flight or re-routing are adequately addressed. The extent of adequate care will have
to be assessed on a case-by-case basis, taking into account the needs of passengers in the
circumstances and the principle of proportionality (i.e.: according to the waiting time).
The price paid for the ticket or the length of the inconvenience suffered should not
interfere with the right to care (Interpretative Guidelines Point 4.3.2).
According to the Regulation, the air carrier is obliged to fulfil the obligation of care even
when the cancellation of a flight is caused by extraordinary circumstances, that is to say
circumstances that could not have been avoided even if all reasonable measures had been
taken.
The Regulation contains nothing that recognises a separate category of 'particularly
extraordinary' events, beyond the 'extraordinary circumstances' referred to in Article 5(3)
of the Regulation. The air carrier is therefore not exempted from all of its obligations,
including those under Article 9 of the Regulation, even during a long period. Passengers
are especially vulnerable in such circumstances and events.10 In exceptional events, the
intention of the Regulation is to ensure that adequate care is provided in particular to
passengers waiting for re-routing under Article 8(1)(b) of the Regulation.
(from Commission Notice -Interpretative Guidelines on EU passenger rights regulations in the context of the developing situation with Covid-19, section 3.3.)
