How do I calculate the required time between London Terminals?

Question

According to BR Times, the Minimum Connection Time between London Paddington and London St. Pancras is 20 minutes from 0700 until 2359 or 25 minutes otherwise. It also notes the minimum connection time is 15 minutes at London Paddington and 15 minutes at London St. Pancras (although for London International it notes 35 minutes). How do I calculate my minimum connection time from — for example — arrival into Paddington to departure from St. Pancras International, such that my London International ticket guarantees me a spot on the next available Eurostar should I miss mine?

Is it:

• 20 minutes transfer + 30 minutes check-in = 50 minutes, or
• 15 minutes Paddington + 20 minutes transfer + 35 minutes London International including checkin = 1 hour 10 minutes, or
• 15 minutes Paddington + 20 minutes transfer + 15 minutes St. Pancras + 30 minutes check-in = 1 hour 20 minutes, or
• 15 minutes Paddington + 20 minutes transfer + 35 minutes London International + 30 minutes check-in = 1 hour 40 minutes,
• or some other combination?

For example, on a Sunday morning, the first train from Reading to London Paddington (GW3719) arrives to Paddington 07:21. The first Eurostar (9010) departs 08:19, implying check-in closes at 07:49. That's 28 minutes after arrival in Paddington, which is tight but doable if the trains are on time (and Eurostar do not always close checkin exactly 30 minutes before scheduled departure; they may tolerate a little later if they're not too busy). Perhaps not surprisingly, this connection does not show up on Eurostar.com which means catching the 08:19 is not possible on a Sunday when starting from Reading. On a Saturday, the Eurostar departs 08:31, and Eurostar proposes arriving to Paddington at 07:01 (GW2522), 1 hour 30 minutes before departure. This is less than 1 hour 40 minutes but more than 1 hour 20 minutes. Would that mean that if I take the train arriving into Paddington at 07:14 (GW4226), 1 hour 17 minutes before Eurostar departure, that my connection is not guaranteed if my train is late?

In some cases, buying a ticket to London International separately may be needed, or one travels a different route than indicated by Eurostar (like for EAR-LNE where Eurostar suggests via WAT, but PAD is faster), or perhaps one connects to the Dutch Flyer rather than to Eurostar, so it would be useful to know the exact cutoff for transfer from one London terminal to another, to have a guaranteed connection.

Answer 1

Firstly, you need a CIV ticket for the mainline railway (the destination should have "CIV" in it) to have a guaranteed connection onto Eurostar at all. If you've booked it all through Eurostar you should have a CIV ticket, though it's also possible to buy them separately.

Secondly, the usual rule is:

• Minimum connection time at origin station
• Transfer time applicable to the time at which and the mode by which you're travelling
• Minimum connection time at destination station

Now, since Eurostar advertise a check-in time of 30 minutes (for standard class on weekdays) for St. Pancras, 15 at Paddington + 20 transfer + 15 at St Pancras + 30 check-in would be the figure I would have come up with myself. I don't have much evidence that this is the case, though.

As for "London International", it's a location for ticketing purposes rather than a real location so I'd be surprised if you have to use them time for that (though it wouldn't surprise me if, in practice, some booking engines use that time). I'd hazard a guess though that the time is 35 minutes to allow for a compromise minimum check-in time, bearing in mind the figure differs for standard and business class, and between days!

Since I don't know for sure I've taken the liberty of reposting this question to Railforums, hopefully someone there will know for sure!