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.
The Severn is the longest river in Britian, it is 354 km from source to mouth. The River Severn is the longest UK river, it's 220 miles long (or 354 kilometres).AND BOO BOO!!