Article 5 of EC/UK261
1. In case of cancellation of a flight, the passengers
concerned shall:
(a) be offered assistance by the operating air carrier in accordance with Article 8; and
(b) be offered assistance by the operating air carrier in accordance with Article 9(1)(a) and 9(2), as well as, in event of rerouting when the reasonably expected time of departure of
the new flight is at least the day after the departure as it
was planned for the cancelled flight, the assistance specified
in Article 9(1)(b) and 9(1)(c); and
(c) have the right to compensation by the operating air carrier
in accordance with Article 7, unless:
(i) they are informed of the cancellation at least two
weeks before the scheduled time of departure

Article 8 states:
Right to reimbursement or re-routing
1. Where reference is made to this Article, passengers shall
be offered the choice between:
(a) — reimbursement within seven days, by the means
provided for in Article 7(3), of the full cost of the ticket
at the price at which it was bought, for the part or
parts of the journey not made, and for the part or parts
already made if the flight is no longer serving any
purpose in relation to the passenger’s original travel
plan, together with, when relevant,
— a return flight to the first point of departure, at the
earliest opportunity;
(b) re-routing, under comparable transport conditions, to their
final destination at the earliest opportunity; or
c) re-routing, under comparable transport conditions, to their
final destination at a later date at the passenger’s convenience, subject to availability of seats.
2. Paragraph 1(a) shall also apply to passengers whose
flights form part of a package, except for the right to reimbursement where such right arises under Directive 90/314/EEC.
3. When, in the case where a town, city or region is served
by several airports, an operating air carrier offers a passenger a
flight to an airport alternative to that for which the booking
was made, the operating air carrier shall bear the cost of transferring the passenger from that alternative airport either to that for which the booking was made, or to another close-by destination agreed with the passenger.

https://eur-lex.europa.eu/resource.h...C_1&format=PDF