Let's put it in perspective.

As folks might have noticed, we've had a bit of moisture locally of late.
The ubiquitous "They" told us that there had been 300+mm of rain in 24hrs and Seathwaite (Up Borrowdale) had something like 342mm.
Consider this. What falls in Borrowdale hurtles to the sea via (eventually) the river Derwent.
Borrowdale miles wide, the Derwent (maybe the same number of) metres wide. Derwentwater is already FULL, Bassenthwaite Lake likewise.

We got it light.
The above has stood in the garden for many days, in the water is a 300mm rule.
Think of more depth than that, spread evenly over most of the Lake District, then channelled into a few narrow valleys.

If you can suffer it there's a 10 min flight over the area here.
The valley at the very beginning has a stream running through it which a fit person could clear at a standing jump.