Common Blue (Polyommatus icarus)
Flight period is one indicator, Common Blue are 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 species tend to be more 'time limited'.
The Common Blue is certainly more common than most other species of blue (unless you are in Purbeck where the Adonis is now as common, if not more so) and so is the most likely one you will see.
The Common Blue likes rough, open ground (especially chalk downland) where they can find an abundance of clovers, medicks, trefoils, restharrows and other leguminous flowers whereas as 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; larger than most of the other common species but a little smaller than the Adonis. Remember too, that the Adonis has a much more vivid blue colouration.
The Common Blue is certainly the most common blue but a closer look is needed before you dismiss a rarer species as a common!