Rutan16, there absolutely is an obligation to offer you transport on another airline if it's reasonable to do so under the circumstances versus the alternatives your original airline can offer. If you're in any doubt on this, please contact the CAA PACT people and ask them to confirm it to you.
EU261/2004 has been fully brought into UK law after Brexit and the only change is that the fixed compensation amounts were converted into UK£ instead of EUR.