An interesting question appeared here on StackExchange Travel. To summarise, the person is buying a through ticket from a UK station to Eurostar, which would have a guaranteed connection as it would be issued to London International CIV. How much time do they need to leave to cross London to have the guaranteed connection? My interpretation, as I've answered, would be the usual rule (origin minimum connection + transfer time + destination minimum connection), plus the minimum check-in time (30 minutes for standard class on weekdays). But is there an official way to calculate it? This is further complicated by the fact that "London International" has a minimum connection time of 35 minutes in the system, along with much longer tube transfer times.