You can’t* actually have a picnic in England unless you have a gigantic custom-made plastic sphere (with air holes) fabricated to ward off the rain. A flame thrower may do in a pinch – aim it overhead so that it evaporates the moisture.
While you are waiting for the rain to cease you can spend lots of time online looking at picnic related merchandise**. Or you can take the train to France.
* This does not stop most English people as hope springs eternal and they erupt like prairie dogs the second the sun peaks from behind the clouds. Blankets are spread, Marks and Spencer sandwiches unpacked, bags of Coronation Chicken crisps opened, and probably some of those nice (?) M&S cans of Gin and Tonic consumed.
** This for instance.