I'm afraid that if you are looking to a UK based company for informal cruises then you may find that the choice is very limited. However I understand that Thomson's are very active at the moment and have acquired Island Star as well as Island Escape from the shortly to be defunct Island Cruise line. Letters have gone out to customers with 2009 bookings advising of the impending changes to their contracts which are being switched to Thomson. I think that you need to keep tabs on Thomson. There's also Ventura and NCL, but they are big modern ships and have a different onboard culture.
We have just cruised with Thompson in the canaries. It was informal but with one formal night which was entirely optional and dressing as for a smart meal out at home sufficed. No need for ballgaowns and tuxs. but worth attending for the atmosphere ( and free drinks).
we have just come home from a transatlantic caribbean cruise with thomson.my husband was a reluctant cruiser so traveling 6 days on a transatlantic crossing was brave.i must say that we had the best holiday we have ever had .the food was excepional and the entertainment realy was broadway standard.the staff were wonderful and genuinly warm and kind.only one night was formal captains reception ,it was well worth it though what a great experience.here is the proof of the pudding ,i got home from work today and my husband had booked thomson coral islands fof jan 2010.all i can say is i can hardly wait !
Definitely Thomson just come back from New Year in Canaries on Thomson Celebration could not fault it the formal night wasnt too formal just a suit and tie for the dinner is fine. would recommend it.