Your specimen is a Whiteface (Leucorrhinia).
There is only one Whiteface resident in the UK, the White-faced Darter (Leucorrhinia dubia).
White-faced Darter (Leucorrhinia dubia) male.
White-faced Darter (Leucorrhinia dubia) female.
One other Whiteface species has been recorded as a VERY rare vagrant – only 2 records since the 1850s – the Large White-faced Darter (Leucorrhinia pectoralis). This latter is easily distinguished by the large pale spot on S7.
Large White-faced Darter (Leucorrhinia pectoralis) male.