For future reference, this
this Seat61 page should give you all the options for London-Paris by ferry, including how to bridge gaps
For small delays/problems, Eurostar normally seem OK. For very large disruptions they do seem repeatedly unable to cope properly. As a general rule, I suggest keeping the
Europa.eu "Your Travel Rights" handy (they provide an app for Android and iOS for offline viewing). Eurostar need to cover your hotels and food during a delay, which you need to claim back from them after. Not sure about re-routing by plane, but re-routing by train+ferry ought to be do-able but I've no idea how hard/easy it'd be to get them to endorse your ticket for that
If you haven't already, ask Eurostar for a full refund of the ticket due to the delays, and see if you can get them to give you something towards future Eurostar journeys too. When friends of mine have been delayed before by Eurostar for a long time, they've normally ended up with a refund and a free ticket for another time