Working backwards, the first flight into IAD from any UK airport accessible this morning is UA923 LHR IAD 0730 1042
Next working backward, we can see that using BFS, SNN, DUB or MAN do not allow early enough flights directly into Heathrow this morning. That means teams had to fly in last night. I have checked on the alternatives. Manchester as a connection point makes no sense because of the early flight yesterday required from Belfast. There are multiple flights from Shannon or Dublin to Heathrow, but they would require ground transportation to get from Belfast to either one. The logical and sensible approach and therefore the one probably taken was to fly out of Belfast City Airport (BHD). Here is the latest flight last night:
BA1421 BHD LHR 1945 2106
Teams either stayed the night at Heathrow or got a local hotel.