In one sense yes, but BA did serve SCL with 747s (one of the regular SCL 747 captains was on the inaugural 787 flight), it just has to make stop en route somewhere. That's actually a technical possibility with 787s too, though I don't think it's been needed so far. So it is possible to run the service with a 777, for example. If your service was cancelled - and there is no evidence yet of that happening - and if they didn't use another aircraft then you would be offered a rerouting via MAD on Iberia.