Just got home after dropping my partner off at Heathrow.
T5 is utter chaos — there's a queue snaking from one end of the terminal and back again, filled with people trying to rebook flights from yesterday.
This intersects with other queues for check-in and baggage drop, so you get people joining the wrong queue or filtering into the wrong queue when they overlap.
The staff barely seem to know what's going on; there's little or no crowd-control so people keep queue jumping or cutting through the lines in order to get to different parts of the terminal.
When you do actually get close to the check-in desks, they're pre-checking everyone in the queue to make sure they're in the right place and/or their flights are still running. This makes sense, but it's taking forever, so you're stood a metre away from five empty self-serve check-in desks…
This was all at 6am when the majority of flights were still scheduled. I've just got off the phone to my Mrs. and her flight is delayed by a couple of hours, many flights are delayed by a lot more than that and many more have been cancelled. So a lot of people are going to arrive assuming their flights are still scheduled (because that's what they're saying on the news etc) and they're just going to add to the chaos and confusion.
As I was leaving, they had staff at the main entrance turning people away, telling them to come back 90 minutes before their departure time. This sounds ludicrous to me; there's no way people are going to get through that mess, pass security and get to their gate within 90 minutes.
@Haggisman, I'm glad you've got another flight booked but be aware of the above — good luck!