It has been 10 years since retirement so things may have changed. An aircraft on approach cannot descend below 1000ft above the runway if at that point the actual RVR is below the prescribed minima for that category approach. . Otherwise the approach may be continued to Decision Ht even if the RVR falls below that limit as it is now advisory. At Decision Ht the landing pilot must have sufficient visual reference, which varies according to the ILS category i.e. I II or III. If these criteria are satisfied, the aircraft can continue to land. At SOU the ILS is category I, which for most CAT I airports is minima 550m and a DH of 200ft above airfield. AT SOU on runway 20 the minima's are higher owing to a combination of high buildings at the railway yard, runway length and width and approach/touchdown lighting.
Try checking Leeds/Bradford, Jersey and Guernsey for their diversion records! I can recall being stuck in Jersey for 3 days awaiting an aircraft to arrive! Twice I have landed at SOU in B737-400 having diverted from LHR due to poor visibility with aircraft that had been downgraded from CAT III to CAT I due to unserviceability!
A previous post has a good point in that diversions do not happen that often in the overall scheme of things. I would question decisions by certain operators to actually take off from elsewhere, heading for SOU when the likelihood of a go around there is high, especially from MAN, JER or GCI which are so close in terms of flight time that it would be better to wait on the ground until an improvement. That would avoid a lot of unnecessary fuel burn and wasted time for customers plus not having aircraft and crews out of position.
When it comes to opening hours, the restrictions at SOU would not have affected KLM as a 0630 departure from SOU would easily have arrived in AMS within the early morning time window as would the reverse before airport closure. Many other UK airports, some used by KLM, have night closure, including LHR! And these airports are very accommodating, generally, with extensions needed as a result of weather disruptions. These limits are usually the subject of planning regulations by the local authorities and a strict nil operations policy exists following approved extensions. I was once denied landing at ABZ at 3nm from touchdown, they switched the approach and runway lights off, as I would miss the deadline on the extension already granted. 1 minute!! Diverted to Glasgow and passengers transferred by road to Aberdeen airport arriving at 4am! That's it for now.