Did you book direct or via a TA ? It's probably too late now but when I booked a fly cruise the option just said London. I checked with the TA and only booked once it was confirmed that flights were to/from Heathrow.
It has worked until fairly recently, there were some broken seats but the breakdown of the planetarium hasn't been going on for years.
It was first reported here last month
From the Terms and Conditions
The Beers, Wines & Spirits Option may be pre-purchased up to four days prior to boarding the vessel and is offered onboard on the first 2 days of your voyage.