corporate-wage-slave

It's a very good question but in a way there is very little you can do other than research the options - one exception coming up though. If it's totally clear that you won't make the connection then BWI may rebook you but it's such a small station that I suspect they will prefer to get you to London. If there are mass irrops (typically weather related) stations will be advised to route people directly. Otherwise it's likely that in LHR they'll route you via ATH or Lufthansa to get you there. JTR isn't exactly a big place, so if you call the hire car people and explain the problem I would expect them to be flexible. And you will have travel insurance.... Personally in this scenario I would book a hire car for the day after landing in JTR anyway, and use a taxi for arrival.

Here is the exception: Now the only people who are going to be helpful here (given this specific scenario where you are miles off the joint business sectors) are airport staff, so if you think / know something is going to happen then don't waste your time calling up. However if you see a late flight from LHR to BWI you can be reasonably sure what will happen to the arrival back to London. So if there is going to be a problem the one area where you can be proactive is arrive at BWI as soon as check-in opens (I think it's 3 hours there, but it will say in Manage My Booking / Other ways to check-in) and suggest the IAD route - or indeed suggest that you hop on the train to EWR where EK run direct flights to ATH. The ground agents will rebook this, the contact centre staff will not. Forget the Icelandair options, BA won't rebook on FI.