You have a Skimmer (Orthetrum) species, of which the UK has 2 residents.
The mature male of both Skimmers develops powder blue pruinosity which causes their appearance to change dramatically from immature specimens, obscuring the ground colour and black markings. At maturity their pterostigmas are very different from each other and distinguish them nicely. Also, only one mature male Skimmer develops a black tip to the abdomen.
Immature males and females look very similar to each other in both species. Happily the dark abdominal markings are sufficient to distinguish the species.