OP's contract with BA is very clear that he must fly all segments in the order issued and that the failure to do so results in the cancellation of all remaining segments. Those segments then retain whatever value they retain under the fare rules of the ticket. I take it that in this case, the fare rules which OP agreed to result in no value.

German courts have ruled that a carrier may not require this provision and accordingly, if one purchases a ticket originating in Germany, one may purchase a ticket which does not result in knock-on cancellation. The court decision was quite clear that the carrier may charge more for such tickets and indeed carriers do. Similarly, one may purchase a full fare YY (the old IATA version) ticket. Those tickets retain their value for a year. Other than a small administrative fee to reissue the ticket, while OP's reservations would have been cancelled, he would have had the ticket value to apply to his new tickets or he could have sought a refund and purchased tickets on any carrier. It goes without saying that such tickets are very expensive and a poor value if all one is trying to do is effectively insure against not turning up on time.