I cannot take the direct flight, discussed that at length. Stuck with a connection.

Not booking the Eurostar. Way too many things to go wrong. 5 trains + walking between St Pancakes and Euston means too much space for error.

My question really centered around what happens if it all goes wrong on the day. The answer is: contact BA and then find your own alternative and sort out the whys and the wherefores later on.