Blues can be tricky chaps to sort out; the silvery underside with orange dots is a familiar feature amongst many of the family. You have to take various factors in to account when separating them.
The common blue (Polyommatus icarus) is on the wing from early June right through until late October as they have more than one brood which overlap giving an almost continual presence during the summer and early autumn. Other similar species tend to be more limited in their flying season.
The common blue is certainly more common than most other species of blue (unless you are in Purbeck where, in places, the Adonis blue is now as common, if not more so) and so it is the most likely one you will see. It likes rough, open ground, especially chalk downland, where they can find an abundance of clovers, medicks, trefoils, restharrows and other leguminous flowers; other species tend to be a bit more restricted in their preferences. In good years, population wise, it is not uncommon for this butterfly to find its way into gardens.
Finally, size can help too; the common blue larger than most of the other frequently encountered blue species but still a little smaller than the Adonis blue. Remember, too, that the Adonis has a much more vivid blue colouration.