I assume you mean evidence of how Greta got to London, not evidence of the existence of Eurostar trains...
No, I don't. She's a private individual, and their travel itineraries aren't normally matters of public record. Do you have evidence that she flew to Heathrow?
We're both just guessing, but given that she very publicly rejected flying from Europe to the USA a while back, and that the "non-flying" journey between mainland Europe and London is a lot simpler than sailing across the Atlantic, I think my guess that she took the Eurostar is much more likely than your guess that she flew to Heathrow.