I'm still reading Patrick Barkham's excellent 'The Butterfly Isles' and mention has just been made of butterfly swarming. Apart from his own incredible experience with Heath Fritillary (1300+ at Blean Woods!), he mentions examples from the golden age of the Victorian gentleman lepidopterist, when clouds of butterflies blotted out the sun - really.
I too have been lucky enough to witness such spectacles:
9 August 1978, Beddington SF, Surrey
Thousands of Small and Essex Skippers emerged en masse across the heavily vegetated sludge lagoon banks, creating a golden orange haze at waist height.
24 July 1992, Middleton, West Sussex
As I drove alongside a large field (crop unknown) I was staggered to realise that the white mist hanging over it was in fact tens of thousands of 'white' butterflies. I reckoned that they were all Large and Small Whites and it was simply staggering.
28 July 1997, Bude, Cornwall
A walk along the cliff top heading north revealed thousands of Gatekeepers and Meadow Browns. I have made this same walk many years since but have yet to see such numbers here again.
28 August 1997, Cavenham Heath, Suffolk
1500 Small Tortoiseshell were shoe-horned into an area of gorse and heather no larger than a football pitch.
6 August 2006, Braunton Burrowes, Devon
A simply stunning day. There were butterflies everywhere, great clouds of mixed species being flushed as I wandered the southern section of this area of vegetated sand dunes. The standout count was of a minimum of 4,500 Common Blues, but four figure counts of several other species made the experience unforgettable.