Southampton will have a new runway length of 1,887m with the new extension.
Other airports in the UK with similar runway lengths:
INV 1,887m
NWI 1,841m
BLK 1,869m
SEN 1,856m
BHD 1,829m
Can't think of any others?
Interesting that they all have similar regional type services. This illustrates that the extension is not really a threat at all to BOH and both should be able to continue operating in a complimentary fashion to each other long into the future.
It's essential for SOU to get their extension done fast in order to prevent its existing operators from having to resort to payload penalties with wet/windy/hot conditions. It will unburden easyJet on GVA which at times operates with payload penalties; likewise Loganair with their ERJ-145's which at times have payload issues. I am unsure of any BA Cityflyer payload issues - I would doubt they have any since they operate from LCY with a runway length of just 1,508m!