If you're on a connecting flight with a tight interval, the system will often give you a forward seat so you can get off first.
Always check both full service and LCC prices, don't automatically assume LCC will be cheaper as they may have filled up rapidly and only have high fares left.
BA seem to be going the way of AerLingus, which have so many add ons that should be included with a full service fare that there is little reason to choose them over a low cost.