I think it is sound practice to know your best alternative flights in advance. If the 216 cancels you will likely know it early on the morning of your travel day, as the outbound 217 will not leave Heathrow. So, plenty of time to work the phones. You might write these flights down in advance:

BA292 departs IAD at 1045p
IB6132 (AA8730) departs IAD at 435p for MAD, connecting to AA8652 dep MAD 910a for LHR
AA5630 departs IAD at 544p for CLT, connecting to AA732 dep CLT at 818p for LHR, or AA734 dep CLT at 1129p for LHR

Always better to tell BA what rebook you want than wait for BA to be inventive on your behalf. And before disruption starts, it is always good to prioritize in your own mind which is most important: to get there as close to your planned schedule as possible, get there on BA, or get there in your originally booked class of service.