This is a very difficult and I'm sure stressful situation for all those that it affects, and my sympathies go out to them all.
Whilst a very useful post, just a slight word of caution though.
As stated at the beginning of the quote above, what followed relates to package travel.
A package is when a consumer buys more than one element of a holiday together, e.g. flights + accommodation, or in this case flights + cruise, but it could be other elements, not including the flights that make it a package. (e.g. Cruise + excursions - see What is a Package Holiday? below).
In this case, Celebrity appears to be offering alternative arrangements for those who have booked flights and the cruise directly with them, so not an immediate issue there. The issue and point of some of the discussion here has been where the flights have been booked independently and here it becomes a bit more complicated.
The above link and quote comes from The Package Travel and Linked Travel Arrangements Regulations 2018.
These came about because of changes in the way people book their holidays, going away from a traditional 'package' and moving to, for example, situations where you may use an online travel service (e.g. Expedia - many others are available), to book the separate elements from different providers and combine them as one booking.
ABTA, the UK travel trade association has an article that provides a bit more of a user friendly explanation here https://www.abta.com/tips-and-advice/is-my-holiday-protected/new-package-travel-regulations
Some quotes here though:-
The new regulations only apply to holiday travel arrangements booked on or after 1 July . If you have booked a package holiday before 1 July, your package holiday will be covered by previous regulation. Package holidays that are for less than 24 hours, or are for business travel, may not be covered by the regulations.
More holiday travel arrangements will be classified as packages. A ‘ready-made’ holiday (this is usually where you book through one company and pay one price) will still be classed as a package holiday. But so too will holidays sold in other ways – for example, many tailor-made trips and shopping basket type sales on websites (where you select the different elements such as flight and hotel).
Package holidays offer the best form of protection. Financial protection means you are entitled to a refund or to be brought home if necessary should the travel company organising your package go out of business. You’re also protected if elements of the holiday aren’t provided as required, for example the right to a refund if bad weather means your holiday can’t go ahead.
Linked travel arrangement is a new arrangement under the Package Travel Regulations. This is not a package and the level of protection is significantly lower than if you bought a package holiday. It comes with limited financial protection in case the company that sold it goes bust, but complaints about each holiday service will have to be taken up with the individual suppliers.
If you have booked your travel arrangements separately (eg – a flight directly with an airline and a hotel through an accommodation booking website), these are unlikely to have any financial or legal protection under the 2018 Package Travel Regulations. However, you might have certain protection for the individual services in other ways – either through travel insurance or through booking with your debit or credit card. Please check with your providers as levels of protection vary.
What is a Package Holiday?
A package holiday is a combination of at least two different types of travel services, which are listed below:
transport (such as a flight, coach or train but not transfers from an airport)
accommodation (such as a hotel, villa or apartment)
a tourist service (such as a tour guide or a trip to a historical attraction) where this is a significant part of the holiday either because of its value or because it is an essential part of the trip.
It counts as a package holiday if your travel company:
Has asked you to pay a single price through a single payment
Has let you select a combination of services – such as a flight and accommodation – before you agreed to pay for them
Charged you an inclusive or total price for all the services you bought
Advertised or sold the travel services to you as a package or similar term
Sold you one travel service; and then transferred your details, including your payment details to another company which you then booked another travel service through within the space of 24 hours.
I'm not a travel or legal expert and the advice given by nomad098 is good "Would probably be worth a call to Celebrity to point this out and if you booked in the UK stating your next call would be to ABTA".