The outbound flight from LHR was delayed taking off. According to EF it took off at 1759 and is still en route to EWR. The delay code is showing as ZY but I can't find anything definitive to state what that is.
BA0197 states in this post
https://www.flyertalk.com/forum/27133986-post10.html that it's capacity restriction at the arriving airport, so that may still be weather related?