They are honouring your flexible working terms and conditions. A desire to increase your hours to full time is not flexible working, and no one is cutting your hours.
What they have very foolishly apparently done is guarantee four years in advance that more hours will be available, allowing you to increase your hours at that point. Unbelievably stupid thing to do, because, as they have now found, lots can change in a business in four years.
In the event of a restructuring it is not at all surprising that they have realised that they can't guarantee what the situation will be in another two years' time, so actually telling you now that they can't guarantee it, rather than letting you down when the time comes is preferable, in my view.
The question is, what can you do about it? If they have given you a contract variation letter confirming that your hours will increase to full time with effect from (presumably) 1 September 2018 or similar date, then when that date approaches, and they say they won't honour it, you could make an argument that they are in breach of contract.
But I don't think there's a lot you can do now, because a lot can change in two years, and when the time comes they might be able to honour it.
I can't believe they wouldn't at least put some kind of caveat into an agreement like that, about needs of the business changing or something. Ridiculous thing to do.