The reason the aircraft was over it's max take-off weight was only due to the runway length at Southampton i imagine.
Basically the heavier the aircraft the longer it takes to take-off. Also if the runway conditions were damp/wet, windspeed, obstacles on take-off climb-out can all add ore reduce the max take-off weight.
Other issues is that if fuel was allready placed on the aircraft for round/trip or possible the weather at destination not expected to be good therefore extra holding & diversion fuel. This may have all been done before the final passenger/bag weight was provided adding to the aircraft being overweight.
On domestic lo-cost scheduled flights it is very difficult to calculate bags per passenger ratio sometimes it's 1 bag in every 5 passenger and on the odd day it can be one bag per passenger roughly.
In regards to trim issues and being moved about. Weight and balance of the a/c is like a see-saw. The main landing gear is approx the central pivet in this see-saw. As you look at the Dash 8-Q400 aircraft it is naturally sloaping upwards from front to back. This means that the plane is naturally nose heavy. Or trim is Foreward when empty. The issue is that naturally passengers choose to sit closer to the front and maybe a habit for reserving seats together towards the front after being stung by free-seating airlines. It is ideal for passengers to be sat 1/3 in the front and 2/3 in the back of the aircraft.
In an ideal world if the aircraft is out of trim then any seat changes can be made at the gate. However with a check-in closure time of -20 to STD this can become difficult at times. So people who haven't paid to select their seat unfortunatly are the first to be moved.
There is another issue in the equasion that sometimes the paperwork and trim is perfectly legal when the paperwork is handed to the Captain. However he/she may feel that it is too close to the limit and some passengers are required to move to ensure a better trim. Sometimes passengers also get moved because others have not sat in their allocated seats which are stated on their boarding cards.