A friend of mine used to work at Bournemouth as an Air Traffic Controller.
Bournemouth regularly hosts 747s. In fact a while back they had a scheduled service to the east coast of the US but I cannot remember which destination or which airline.
Southampton airport and Bournemouth airport have to co-operate very, VERY closely over the airspace control because they are so close together AND because the wind direction often means that the aircraft are taking off TOWARDS each other.
I visited him in the tower once, and when he was on radar control rather than visual duty, he was permanently on the phone to his counterpart at Southampton.
So seeing such a plane over Soton doesnt surprise me.