Pottwiddler
If there was 'fantastical bridge OVER the M27' but this will NEVER happen especially due to the water levels in that area!
The runway at SOU can not really be extended at the southern end even if there was a bridge over the M27 arriving aircraft would still have the hazardous approach to contend with and would only therefore benefit aircraft taking off from that end of the runway.
The only way they could realistically extend the runway at SOU is at the northern end by at least 450 feet.