Copied from Dog World's FB page:

For anyone travelling to Windsor:

Exhibitors travelling to Windsor ch show weekend 30th June/1st July, be aware that the M1 is closed between junctions 13 and 11 at 8pm on 30th June, to noon 1st July, due to the demoliton of a bridge. Diversions will be in operation but are long. Please allow plenty of time for your journey.

This is a total closure not just one way.