Anywhere !... Almost all UK rivers have some access points to the riverbanks, that are usable by the general public.
The Ants.....
Picnic is usually used as a noun:We are having a picnic on the weekend.But it can be used as a verb:We will picnic beside the river.
A Picnic Disturbed - 1904 was released on: UK: November 1904 USA: December 1904
The river Thames is the largest river in the UK.
There is no 'river UK' !
As far as I am aware, there is no river Drake in the UK.
You would be in Virginia.
Rat and Mole had a picnic on the riverbanks in "The Wind in the Willows." They enjoyed their picnic of sandwiches, cold chicken, cheese, and sardines while watching boats go by on the river.
Kipper - 1997 The Picnic 3-3 was released on: UK: 1999 USA: 8 March 1999
In Kenneth Grahame's "The Wind in the Willows" Mole and Rat picnicked on the river bank.
The main river in the UK is the River Thames. It is the longest river entirely within England, and it flows through the capital and most populated city in the UK, London.
The River Severn The River Severn