With the large cruise lines you will find the bar charges are pretty much the same. Bar fees on Carnival, Royal Caribbean, NCL, Celebrity, MSC, and Costa don't vary much. However most cruise lines will have special pricing for the drink-of-the-day or they will offer things such as buckets of beer (4 or 5 bottles/cans in a bucket of ice) where it is cheaper than if you bought them individually.
When you take cruises during school holidays you have to expect higher numbers of children aboard. Not much you can do about that, but as Cruise Fanatic notes it probably isn't as bad in the Med.
Every day there are cruise ships with families of four in a standard cabin. It is certainly a tight fit and one I am not willing to do, but the flip side is that you are really not in the cabin that much. Envision a roughly 11 foot by 15 foot box that also includes the furniture, bathroom, and closet. Perhaps you have an area in your home that is close to that size. Now picture your family in it all at once and see if you can live with it.
Rules for cruise bookings are often different for UK citizens, so if you read something on this board remember it may be someone from Canada or the U.S. "talking" and our booking rules are different. For instance I believe it is common for your deposit to be non-refundable where my deposit usually is refundable within limits.
Penny is correct. You really need to find a good agent who specializes in cruises. You need an expert who can address your expectations and do their best to match you to the ship/cruise line that will offer you the best personal experience.