As there are thousands of them it is impossible to answer here.
It is the river Shannon in southern Ireland .
River Trent River Thames River Severn
Geographically, the British Isles are Britain and Ireland and all of their off shore islands. The longest river in the British Isles, at 240 miles, is the River Shannon in Ireland.
Shannon Severn Lee Trent Lagan Tyne Corrib Thames Liffey Mersey