Minimum cross-London connection time for Eurostar

[Muzer]

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.

 

[CyrusWuff]

Connections involving Eurostar have always been 35 minutes on top of the transfer time to/from Waterloo/St Pancras due to the minimum check-in time and potential for passport checks on arrival into London.

The table that used to appear in the National Rail Timetable can be found in this post in the forum's Fares & Ticketing Guide. This has since been superseded by the notion of having different connection times at different times of day, but is generally within a few minutes of current times.

For example, the connection from Liverpool Street to St Pancras is given as 41 minutes in the table (15 minutes for the connection time at each end, plus 11 minutes for the Underground journey between the two) and you then need to add another 35 minutes for the Eurostar factor, giving a 76 minute minimum connection. Looking on BRTimes.com gives this as being the same today for most of the day, but says you should allow 21 minutes for the transfer between 0001 and 0529, and 16 minutes between 0529 and 0659, and between 1901 and 2359.

 

[Muzer]

That's pretty much what I expected, then (minimum connection time of each station, transfer time, and check-in time added together; I just got the check-in time wrong); I'm just curious, though, where does 35 minutes come from? Is there anywhere (besides the minimum connection time for London International or whatever) that codifies this 35 minutes?