I think with a UK booking there are some rules they have to follow to do with major and minor changes.
With major changes I think they have to offer full refunds, a bit like the 10%/2% rules for price increases.
I have a first edition caribbean 2008-2009 brochure(princess) handy
I think this allows for full refund within 14 days of being notified of the significant changes in tineraray. clause 26.and compensation(cl 27) over the refund if at short notice <56days 5% increaseing to 20%14days or less before departure
If a fly cruise it should be covered by the ATOL licences.
After leaving it also allows for compensation(which I think means partial refund) and transport home for significant changes(cl28).
obvious safety reasons like adverse weather are excuded one departed.
On-line seems to be broken so I cannot find any information there.