As you are in the UK what about a Baltic cruise? Its one of the best we've ever done and the airfare should be reasonable. Typically there are tow formal nights on this cruise but you can choose to eat elsewhere like the buffet area then. IIRC NCL has no more formal nights so this may be a good choice for you.

Oceania has no formal nights. It's our favorite line.

We're in mid 60's, and Carnival is our favorite. Have done some other lines, and like them, but Carnival gives us best prices and has the roomiest cabins. I'm sorry I don't know if this is an option for UK, but worth looking into. As for the casual dress, even on elegant night, my husband in slacks and collared knit shirt was fine in the main dining room. The buffet on lido is always casual. Sometimes see tuxes and evening gowns on elegant night, but most passengers do not dress up that much. Good luck.

Defineitly east Med over Carribbean if you are from the UK. The air fare would make all the difference.

How about a Trans-Atalntic?

When can you go? Would you prefer a European cruise as your first cruise? We had 2 excellent cruises - the best was Norwegian fjords, the 2nd very close to the best was Greek Islands out of Venice.

As Jacketwatch says the Baltic is a great place to cruise, we had a wonderful time with some good shore excursions except for one incident. We were on Cunard, the morning coach broke down so we returned to the ship with about five minutes to spare before the afternoon tour. We spoke to the Cunard rep on the wharf explaining that we had missed lunch, she told us that was our problem and Cunard does not accept any responsibility, she would not do anything to help us at all.

I have taken about 10 cruises between Royal Carribean and Carnival over the years. In my younger years I have gone with friends and shared a cabin with 3 people in both interior and ocean view cabins. Later my husband and I have gone and had ocean view and balcony cabins on various levels within the ship.
I will say that the balcony is nice if you have multiple days at sea when you will actually have the chance to use it. If you take a cruise that has stops in a port every day and plan to get off the ship the balcony in my opinion the balcony is not worth the extra cost over oceanview. I will say DO NOT book a cabin across the back of the boat because the anchor noise when docking and departing is somewhat loud in the cabins.
We get off at every port because our personal goal is to see as many cities / ports as possible. We split our ports between booking shore excursions through the cruise line and doing our own thing. We usually book a snorkeling outing when in the carribean thru the cruise line and potentially a swim with dolphins type excursion. In St. Thomas or Jamaica for example we have gotten off the ship and negotiated with a local taxi driver to see the whole island with just the two of us. In Cozumel we found a private beach option online for $50 pp and it included food, drinks, beach chairs, umbrellas, floats, etc and they only accepted 50 people which was nice to not be on a packed public access beach.
Every cruise line offers almost non stop options of activities so you can do as much or as little as you would like while on board.
We have mostly cruised in Mexico areas and Carribean (we are located in the U.S.) - we are taking a Baltic Cruise in July out of Stockholm on Royal Carribean.
I hope some of this information helps

If you have never cruised I would recommend a short and inexpensive cruise as a first cruise, just to see if you like cruising. No sense in taking a long and expensive cruise just to find out cruising is not for you. I would save that for your second cruise.

Always choose a cabin that is sandwiched between cabins above and below, nothing worse than spending a week below pool, showroom and restaurant venues with the resultant noise in the early morning and late evening.

Our favorite line is Princess, it finds just the right balance between fun and stodgy, and has some of the best food among the mass market lines.

As to casual, most lines today offer "Anytime" dining where you can dine at a time that is convenient to you and with whom you choose or alone as in a regular restaurant. Most lines have a "Formal" or "Elegant" night or two but you wouldn't believe what passes for formal or elegant these days. Of course you can always eat at the buffet on those nights if you choose, I wouldn't let that be a determining factor regarding your choice.

Enjoy your cruise.

Regent is fabulous and small just like Oceania. we prefer smaller ships like these (500-700 people) but you pay alot more. Spending my vacation with 1500-2000+ people and waiting in lines, rock climbing, skeet shooting are not my ideas of time at sea. that being said, you really have to evaluate your self - do you like to be with crowds or do you prefer smaller more intimate groupings. - what is your budget? Can u afford smaller? taking a cruise on a large liner just to see if you like it would not work for me. we have tried that and said NEVER AGAIN for cruising and then we discovered Regent and we were hooked on smaller ships. Regent and Oceania are resort casual - not dressy where you have to wear tux and gowns. I have a friend who goes on larger ships and she and her husband have room service on the formal nights. BALCONY is a must for me - extends the living space and it is so nice to have drinks or breakfast on your balcony. good luck in deciding......I know regent has a cruise that goes down the Adriatic from Venice - with some great ports of call.......