According to an article in the BTO magazine (issue 335 Summer 2020) by David Smallshire from the British Dragonfly Society the top 15 species most likely to be encountered in the UK are (in descending order):
1 Blue-tailed Damselfly
Duchess Pond, Stoke Park Estate, Bristol – 2nd June 2020
2 Common Darter
Duchess Pond, Stoke Park Estate, Bristol – 17th August 2020
3 Common Blue Damselfly
4 Large Red Damselfly
Male – RSPB Strumpshaw Fen – 17th June 2019
5 Azure Damselfly
Duchess Pond, Stoke Park Estate, Bristol – 2nd June 2020
Duchess Pond, Stoke Park Estate, Bristol – 8th May 2019
Duchess Pond, Stoke Park Estate, Bristol – 17th August 2020
6 Brown Hawker
7 Southern Hawker
8 Emperor Dragonfly
Duchess Pond, Stoke Park Estate, Bristol – 2nd June 2020
9 Migrant Hawker
10 Banded Demoiselle
RSPB Strumpshaw Fen – 17th June 2019
11 Four Spotted Chaser
Male – RSPB Strumpshaw Fen – 17th June 2019
Female – Strumpshaw Fen – 17th June 2019
12 Emerald Damselfy
13 Broad-bodied Chaser
14 Black-tailed Skimmer
Duchess Pond, Stoke Park Estate, Bristol – 2nd June 2020
RSPB Strumpshaw Fen – 17th June 2019
15 Ruddy Darter