My partner and two young teenagers are traveling to the UK in June. From London we travel to Colchester for a 3-day biking trip. From there we take a train to York. I understand we must travel back through London to get to York. Here is my question: we come in to Liverpool Street Station and go out of Kings Cross. It looks like 15 minutes by tube between stations but with luggage and kids, how much transfer time between trains should we allow? Is 40 min enough? We do not travel by train enough to know how much lead time we need. Thank you!