For Kiwi, original poster, your "short trip, warm climate" will be tricky from UK, as the warmer climes are usually in the Med, and that's extra time to get there and back??
Quite a few lines go to the Fjords, or Baltic capitals, they are usually about a week, and in our summer, Scandinavia is a perfectly reasonable temp.
We did a lovely cruise, from Soton, went across the top of Spain, called at La Coruna, Gijon, Santander and the port for Bilbao, then back to Soton. Warmer weather, not too long. I think that was a Fred Olsen cruise (smaller ships, and a line geared to the more mature British customer!) - but their schedule for this year seems to have no such cruises. BUT they DO some very similar ones from Liverpool, and others to the Fjords from Newcastle, so IF you're northern based, could be ideal?
Several comments have suggested Saga, who are enormously expensive, but build in a ride to the port (you may have to share that ride of course!).
If your budget can run to the Azamara line (highly recommended) their ships are all smaller, and they won't always do the massive cabaret shows found on larger ships, less onboard shopping opportunities too (good!). I found a trip from Lisbon (so yes, a dreaded flight, but it had a couple of nights there), called at several Portugal places then some in Spain.
I'd also recommend the IGLU Cruise website, we've used them quite a few times, always good service.
If you actually want larger, busier ships (& some folk do) I'd suggest Celebrity, who run some trips out of the UK. Masses of reviews and advice can be found on the Cruise.co.uk website.