We are doing teh same trip in July. We researched the transfer times, but to be on the safe side, we simply booked teh Premier Inn at Kings Cross for the night at jusy £80 for the family. We are heading to South Wales for a weeks holiday before heading back to Ireland....will need to recouperate after the Disney madness!!!!
A taxi between the two stations was quoted to me at about £12-15, but then again, you are hoping that traffic isn't a problem. What I do know is that Eurostar now do a connecting service from a range of cities around the UK. So if you book a through ticket from say Cardiff to DLP return, Eurostar are liable for any missed connections. Well, at least thats what the kind lady told me. She said that they are responsible for getting you to your destination provided you booked the whole journey with them......unfortunately this service did not extend to where we need to go, but might suit you??????