Maybe they're taking into account that you did travel to Rotterdam? How much money that should represent is debatable of course, since they don't have a fare for that.Why would Eurostar be offering me a refund if only £140?
Only bus/coach and train options are covered, according to the applicable EU regulation, plus accommodation if you get stranded overnight and meals/refreshments as appropriate if your delay is at least 1 hour.If the train had been ‘just delayed’ I would have received a healthy fraction of the ticket cost back. But because it was cancelled I was clobbered with extra accommodation, subsistence, Dutch rail and Stena Hook-Holland costs. I have insurance, but have not yet started that claim or looked at the excess on it. Should Eurostar be coughing up?
If there were rail options to get to London the same day after stranding in Rotterdam (normally via Utrecht - 's-Hertogenbosch - Breda), then those should have been used... but if Eurostar didn't propose that, despite their obligation to do so, and rebooking was not possible, then you could argue that you had to organise something yourself. I'd start with claiming back all additional costs from Eurostar. You should however not ask to refund the original ticket, since then you opted not to travel, the original contract doesn't exist anymore, and Eurostar don't have any obligation anymore. If on the other hand they didn't propose all mandatory options, then you may have grounds to argue.