My holiday company has changed my accommodation. Can they do this?

This depends whether your holiday company specifically states in their booking conditions that they can make changes like this. If they don’t reserve the right to do so, then any change will be a breach of contract. If they do reserve the right to make the changes, then they must follow what their booking conditions say.

Our Code of Conduct states that booking conditions should clearly advise that if a significant alteration is made, the travel company must offer an alternative if available, or a full refund. The company might have to offer compensation on top of a refund if it made the significant alteration after your balance due date had passed. However, compensation is not due if the reason for the significant alteration is unavoidable and extraordinary circumstances. 

Unavoidable and extraordinary circumstances
Such circumstances including war or threat of war, riot, civil strife, industrial dispute, terrorist activity, natural or nuclear disaster, fire or adverse weather conditions.

ABTA's advice
If the change is to accommodation within the same resort and of a similar standard as the accommodation originally booked, it won’t normally be regarded as a significant alteration.