It's not just UK261 - that would probably have got me a rebooking on Eurowings if I'd eventually litigated it - but rather a breach of Reg. 15 (1) and (4) of the Package Travel and Linked Travel Arrangements Regulations 2018, i.e., that the airline is no longer able to fulfil performance of not only the flight but the linked car hire, and this gives right to redress under Reg. 16. The airline may argue that the reason for the increased cost is due to me (the traveller) due to my choice of destination airport (Para. 4 (a) of Reg. 16) but I didn't cancel any flights or instigate the change, other than by declining the refund that they were so keen to push.

(And to defend the agent somewhat, she did say that she'd checked with a supervisor beforehand, unprompted - albeit that's at best a numbers game if the supervisor is not applying the regs correctly).