DP,
50 foot is a minimum transit altitude. This does not mean aircraft can't fly lower during certain procedures (not for discussion on here) so what you saw may well have been totally within legal limits.
Helicopters can and do fly under wires (granted not telephone wires hopefully!)
I think you would have been more concerned if the Merlin
wasn't turning and burning whilst over your house (which you haven't specifically located apart from S Oxon).
Come outside and wave next time one flies over and you might even get a wave back!
