By elimination I think that this is consistent with the Great Orme race of the Silver Studded Blue, Plebejus argus caernensis. Common blue and Adonis blue are simillar BUT this is way out of range for Adonis and it lacks the black lines on the white borders of the wings. Silver studded blue start flying on Great Orme in early June (but alot of butterflies have been late this year); common blue from the second week of May. I cannot see anything which allows me to be certain either way between common and silver studded, unless it was taken early in the year when common is more likely. You could try UKButterflies web site.