Book flights directly, after using third party websites for information. If there’s a cancellation or changes that deserve compensation, airlines in my experience compensate swiftly and making changes is far easier. I also found third parties took a colossal ‘cut’ on monies due back to us.
They’re also more amenable to later change requests I’ve found. I like hotels.com, but their customer services wouldn’t allow me to change to less popular dates, even when the hotel said it was fine.
Was the Eurostar cost very high?
I’ve looked into the London to Amsterdam route a few times post pandemic, crikey it’s pricey compared to cheap as chips flying.