I think that there is a 40 mph speed limit on the M1 following fire but all lanes are open. The beauty of using the M40/M4 is that you come in from the west of London which is where Heathrow is situated. I used the route a couple of weeks ago coming down from Cheshire and there were no holdups or roadworks at all. Plus as you go through 'Cropredy country' you get the added bonus of watching red kites swoop over the road - although that might just be me as I'm a bit of a twitcher!