Weather related or capacity restrictions at EWR, it is still extraordinary circumstances. Any claim for comp. is going to be uphill (and will surely be denied by BA)
Also do keep in mind that it is the arrival time at LHR that is relevant (rather than dep.time from EWR). It frequently happens that fligts depart with more 3 hours delay but arrives at destination (arrives at gate, doors open to more precise) with a delay of less than 3 hours, i.e. no comp.