I have a colleague on this flight so I can shed some light. All the passengers were boarded but then during the pre-flight fumes were discovered in the cabin. Everyone was off-loaded and the crew had to be taken to hospital as a precaution. Then there were some minimum rest requirements which meant that the earliest they can take off is 2300 local on Friday. There's a major trade show in Las Vegas this week that means that all alternative routing is pretty much booked up. Eventually after several calls to the Gold line he managed to join the rest of us on tomorrow's BA2276 to Gatwick.