OK, trip is now cancelled. We decided we wanted to wait a bit longer until we take Lucas, plus it'd be nice to go when the exchange rate isn't so shocking. We are renting a gorgeous cottage in North Devon instead, which I think will be more to Lucas's liking - we'll take him when he understands what it's all about a bit more.
I've contacted Eurostar about getting a refund but I don't hold out much luck as they are 'non-refundable' tickets, darn!
We are finally going back, almost 3 years since our last visit, and now with a 3rd person in our group - how exciting!