We were on Island Star this July and went to the Oasis restaurant on two occasions. It cost £12.50 per person but on most evenings they do a promotion at £29.90 for 2 including a bottle of wine which we found to be very worthwhile. Both the food and service were excellent and well worth the supplement. We also heard very good reports on the Steakhouse restaurant ((£14.95 per person or £34.90 for 2 inc wine) but we didn't try it simply because we preferred the starters in the Oasis restaurant and there was less emphasis on chips with the steaks. From what I recall the wines in the promotion were normally priced at £12.50 per bottle and were very good. The whole evening was very relaxed and you were not rushed in any way. Enjoy your cruise.
I have not been on Island Cruises so cannot comment about the quality or value.
Nowadays most cruise lines have "Speciality Restaurants" and they are there nor for the passenger but to boost the profit so, with the exception of a few lines like Azamara, Swan Hellenic and the like there is a charge made which, in this case, seems high so worth asking about value for money.
The wine need not necessarily be that plonkish as it is probably an incentive to get more people to use the restaurants when they are underbooked. I am sorry that nobody has answered your main point, why not try and ask it again? Regards... Neil.