KARFA

So the easy way to look at it is that once you are present at AMS (let's say that's your exEU starting point) at the right time for your first flight of your exEU booking any cancellations/delays are BA's problem to sort out in the sense that you need to talk to them and they will rebook you and make any amendments to your itinerary to get you back on track and to your destination. You should certainly not take matters in to your own hands, as noted by shorthauldad, and you should not self-reroute and hope to pick up the itinerary at some later airport, this wouldn't work. This is why I say for IRROPS once on your ticket there is less risk since you shouldn't be left stranded, but equally if you know what you want rebooking on it is easier and more likely the agent will give you what you want rather than a rerouting you don't like as much.

If you are not at AMS at the right time for your first flight, for example you booked a U2 flight from MAN-AMS to position, then if something goes wrong with your U2 flight you have a big problem. You are not at AMS at the right time and therefore not BA's problem to sort out as you have not started your exEU ticket yet. There are some ways you can try and get things back on track including appealing to the BA agent at AMS to rebook you when you do arrive late, or calling BA and going to LHR and trying to persuade a BA agent to reinstate your booking starting at LHR. However, you have no guarantee of either working and BA may well tell you no.

So you can see the risk period is basically getting to the start point of your exEU. What you do to minimise that risk can include things like going the day before to AMS.