Additive is used in unleaded and super unleaded which should keep it stable for approximately 6 months.
Temperature plays a big part, if stored in hot conditions you will loose some light ends out of the fuel i.e. Butane as it is low flash material etc.
Its difficult to say where you can fill up from as it depends which bulk plant supplies your area.
Just because it's a Tesco/ Shell garage etc does not mean the fuel has come from Shell direct.
At our Esso depot you see Tesco/ Morrisons/ Shell etc turn up to take fuel, as far as iam aware its only Shell that use an extra additive as they are loading.
There is a golden standard for each grade and most blend in give away for a safety margin to get the tanks on grade so each grade is usually higher in mon/Ron than the pump figure states.