Hi Gaza,
I certainly appreciate your frustration having your bag left behind, it is incredibly annoying.
Hopefully this might answer your query. I suspect, like the Captain mentioned and you have passed on, that the aircraft was over MTOM, more specifically in this case the RTOM (Regulated Takeoff Mass). Whilst with a full load of passengers, sufficient fuel and baggage you wouldn't expect this to be an issue for an aircraft at an airline's main base, however, in icing conditions for the Q400 it can become a problem.
For clarity Icing conditions exist on the ground and for takeoff when the Static air temperature is 10 degrees celcius or below and visible moisture is present or visibility less than 1600m (Fog). As you mentioned it was raining, this coupled with a temperature which is likely to be below 10 degrees at this time of year meant that an Icing departure would be required.
For the Q400 this means our Propeller Anti icing equipment is on for takeoff, which takes away power from the aircraft's engines and therefore degrades the aircraft performance. This isn't particularly noticeable with two engines operating, but all performance is calculated on the assumption of an engine failure. The level degradation in performance depends on many factors, such as wind direction, Pressure, temperature, runway length, up or down slope, airport elevation etc. I have seen an icing departure reduce the MTOM by over 2000kgs in some instances which as you can see leads to quite an issue for a full aircraft which was already close to the aircraft MTOM of 28998kg. Southampton is a relatively short runway therefore performance losses are very likely, Gatwick or Manchester there are likely to be no issues.
The runway being wet itself would not have decreased the MTOM, it would have simply reduced V1, this is the speed at which if we have an engine failure, we must continue the takeoff.
In terms of leaving bags behind, this is frustrating and is something which rarely happens. A combination of factors could lead to this: A full passenger load, icing departure, high required fuel (SOU - MAN is a short hop, however the fuel uplifted will depend not only on the actual weather conditions, but also the forecast weather conditions, so if MAN and diversion airfields were forecasting poor weather such as Fog, then the fuel carried can be quite a lot more than just required to get to Manchester) and unusually high number of bags. Whilst I don't want to be making excuses, I'm just trying to explain how a number of factors can lead to bags being left behind. Why this wasn't communicated is something I can't comment on due to not being there.
I hope this helps a little bit.
