This is being discussed a few threads down, although it’s not an obvious title. The airline know there's difficulties with both the airport and themselves, so they are offering a free reschedule for a future date. It's to relieve pressure in the hope some people choose to book later down the line. The options are clear "you don't have to do anything" and can continue as is.
They often roll this out when there's large scale issues - operationally / IT etc
There is of course, a chance this flight could be cancelled at the last minute, but this message is on all LHR short haul bookings for a couple of weeks to come.
Good idea leaving a buffer pre cruise, it would certainly be wise to cram a few days clothing into you hand luggage to be safe, and perhaps mix others across the suitcases in case a bag or two was delayed