Travelling on Christmas day...

Ok, so its not Virgin, but First to Miami via DUB the night before, but the option of flying 23rd and overnighting in LHR vs overnighting in DUB isn't there. So a first A380 flight on christmas day is a possibility...

I take it the lounges are open in general (T3, so would be same as Virgin terminal). Are the likes of US hotels open? Do the major cities close down on Christmas day? Would we have to stay at airport and eat in the hotel?

Previous outings were planned to be in Vegas on Christmas day, which is clearly not a problem...

It's a public holiday in both countries. Certainly LHR will be business as usual.

US cities like NYC are extremely busy on 25 December, with people shopping, dining out, and going to the cinema. It's a very busy day for most businesses.

Miami is not quite like that, in that many (but certainly not all) restaurants will be closed. The hotels will certainly be open. Christmas Eve dinner is quite a big thing, so I imagine many restaurants will be open for that. I'd certainly book, though, as the places which are open will be very popular.

Many Americans like to dine out on Chinese food on Christmas Day too.

The no. 1 and I used to travel to the US frequently on Christmas Day, him being in retail at the time and all that, and usually we'd get to our destination for Christmas dinner at friends with no hassle.

You're right, LHR will seem to be as normal, because it is. In the US however it can be a little mixed, queues for taxis might be longer, not all restaurants are open as Nylon says, and certainly apart from convenience stores most shops are closed. It all changes back to business as usual on the 26th though.