My wife was doing social support at Heathrow in those days, 9/11 had a huge impact with so many planes grounded and all routes to USofA 'not allowed'. LHR being a gateway airport had many Americans and others in transit to the US. We were also swamped with offers of accommodation from all over the country. The towers collapsed it was mid-afternoon UK, a huge event in the UK too. There was little we could do for you in America, but all those in Transit were going to be helped, somehow. In the airport the social team quickly had Counselling available to all. There was little information available at that time but what we had was passed on. The local town councils opened up what facilities they had and bussed everyone in so they got a meal while overnight arrangements were made and the towns social workers took over there.
In the airport we (I was there to help) were rounding up any passengers still there. I remember one chap in the orange religious robes, looking rather bewildered and with no English. The Social Team were able to help him through the usual interpreters that are available at the airport.
British Airways and the Heathrow management put up huge tents for day time use as people tried to get home some how.
From the airlines point of view the aircraft were now all out of position, or just grounded in America. This affected more passengers of course, but not so badly. I think the US no-fly ban lasted three days, as I remember, which meant 3 days of passengers looking for their flights on top of the already well subscribed passengers numbers every day adding to the queue. Our dependence on Air to cross the atlantic became very clear, yet we've made no changes. No Liners, as in the old days, to keep the lines open. The armed forces are just not equipped for shipping civilians in comfort, and no-one keeps airliners in store to provide additional capacity.
The Icelandic Dust Cloud affected Europe badly. Technically the dust in the engines melted into a polluted glass like product which stuck in a thin film to the blades. Not a problem until the engine cooled down and the glass set. The additional maintenance was the real issue for operators, costing and cutting aircraft availability. What actual damage was being caused was not known, but safety came first so few aircraft flew. Now we have dust radar, on board monitoring, and a better understanding of the maintenance issues. But that Icelandic eruption was 'small' in geographers terminology. Just hope Sami doesn't throw another one in, some where in the world!