There is a 300 mile rule for cancelled services, LHR is just over that (313 miles) but I suspect you will be able to persuade BA to drop that leg if necessary. BRU and AMS are within 300 miles (as are a number of other places). Moreover BA have a Guideline that allows services to be rebooked on Lufthansa Group flight numbers (which may include EW potentially). Lots of detail on this in the main Rebook thread.