Bletchleyite
Veteran Member
Which is why I said that your refund rights could depend on what ticket(s) you hold.
Not, however, which type of CS ticket is held (though some may offer a "no quibble" refund making life easier). It all depends on what is used for the first part of the journey, and that it has validity to Euston specifically (e.g. a ticket to Underground Zone 1, or if there isn't such a ticket then a paper Day Travelcard). It is particularly important that a paper or e-ticket is used, and not contactless, and the OP is very strongly advised to follow an itinerary for the whole journey obtained, and printed, from nationalrail.co.uk.
The OP would also be well advised NOT to walk from St Pancras, but to use rail for the whole journey including a LU transfer, or as a minimum to have a ticket that would allow that. Walking between the two would allow for the argument that they walked too slowly were the CS to be very narrowly missed.