I'm a walker and wheelchair user and find Southampton a very good spot to cruise from. You ask a lot of questions! But here's what I can tell you for starters:
Southampton has several cruise terminals but generally all have a separate boarding line for wheelchairs. It often means you will get to jump the queue a little bit.
Several lines go from there. You can see the entire calendar, complete with some ship details if you look on the website. http://www.cruise-southampton.co.uk/
Some folks might disagree with my description and I have to confess to not having been on all of them, but here is a prejudiced view..................
- P&O are the most "British" of the lines and the only one where you pay in £ sterling. The rest, you pay in US$.
- Princess is silimar but a bit more upmarket and international.
- Royal Carribean International is of the same type, but maybe with a younger clientele.
- Celebrity is a bit more upmarket.
- Cunard is generally regarded as the top of the pile.
- The premium lines like Seabourn are irregular Southampton visitors and usually don't do return trips - you have to fly back.
- Generally, the smaller ships (1700-2000 passengers) are more suited to an older crowd, whereas the mega ships like Ventura, Azura, the Princess ships and the big RCI boats have a younger end and often get filled up later with cheap deals out of the newspaper. Celebrity is probably an exception to this. I'm happy if anyone takes issue with this view, since I've avoided the big ones and am working on second hand inforamation.
Average age profile on board varies, with some of the P&O and RCI ships being quite young. On the larger ships and especially on RCI you will find water slides and rock climbing walls, with a population to use them. At the other end with Cunard you will find daily bridge clubs and a string quartet in the bar. All will have evening entertainment of varying quality but generally including productions shows, a comedian, a singer, a magician, etc.
This thumbnail picture is very superficial, so I suggest you get plenty of brochures, work this forum well and pick a ship and an itinerary that appeals to you.
All are suitable for your MIL, so long as she has someone to help her. You are right to be looking at a wheelchair accesible cabin and these usually book up fast. You might get away with a normal cabin and a shower stool, but I wouldn't recommend taking the risk if you are first timers.
Declare the issue when you book and all lines will ask you to complete a questionnaire in advance of sailing, but just about anything can be accomodated on board such as restaurant seating. Be aware though that they expect you to do the caring - the crew are generally helpful but not specially trained. Some ships will have a disabled coordinator on board - you should ask in advance and seek her/him out but it is not the end of the world if this is not available.
I suggest you find an itinerary which does not have any tender ports, or at least as few as possible. If your MIL can take a few steps then she can go on a tender with her wheelchair to follow, but this is very dependant on weather and you might as well avoid the risk. The Carnaries are good for this and reasonably accessible, but some places (Gran Canaria as an example) have horrendous kerbs even on pedestrian crossings. If there are plenty of you this won't be so much of a problem.
There are several parking options in Southampton - from the people who take your car away at drop off (CPS) to those who are a bit cheaper but keep your car remotely and drive you in. I prefer the former. If you book early then most of the lines have a deal which includes free parking. Another alternative is to find somewhere local to stay the previous evening where they do good parking rates. Some of the big hotels do this, but we know of a lovely little farm and also an inexpensive country manor where they will let you leave your car and drive you into Southampton for boarding.
Hidden charges? Yes, plenty for the unwary. All have a standard daily "gratuity" to add to your bill - this varies so see the brochure. You can cancel this on board but this is pretty mean and bad form so I'd say you should roll this into your expected holiday price.
Some lines add 15% to the price of drinks. Some let you take reasonable amounts of your own wine and spirits on board to consume in cabin. Others (RCI and Celebrity) are very stingy about this and your booze will be sent to the naughty room for the duration of the cruise. Soft drinks can be pricey and if you are on a tight budget you could take plenty on board. Drink prices range from comparable to a pub in the south to a bit of a shocker if you want to hit the cocktails and high end wines.
Excursions are generally about twice the price of what you could arrange for yourself but at least you get the peace of mind of knowing the ship won't go without you (usually). Some lines will offer excursions in wheelchair vehicles but these are not universal. Where not available you will need to stow the chair and help your MIL up the steps on to a coach. You won't be able to find out details of this before booking. All lines have disability help lines in the UK and they are generally very helpful with information but the quality does vary enormously. Persist if the first response isn't great.
Can't help with insurance. You won't be allowed to sail without it, but as you will have found from MSE, the best bet is to use t'interweb thingy and look, look, look. Most companies do not have a problem with a disablilty providing things are stable and there is no current treatment. Being 80 is going to be more of a problem, but you will find cover and I'm sure someone in that age bracket can help. Also, post the question on the Cunard forum - you see plenty of happy cruisers in that group on their ships
The forum doesn't allow travel agent recommendations so any posts will get deleted. The two you have identified get a good rep. We always use a larger one, which advertises as a cruise club and I believe is the largest in the UK. I have never been able to beat their price elsewhere and the service is fantastic. Others will tell you to book direct with the cruise line but personally I've never found any benefit from doing this.
After you book, watch for subsequent price drops. P&O, Princess, Cunard will not let you have the drop but you might be able to get an upgrade or some on board credit. RCI and Celebrity will usually honour the price drop and I've had three BIG reductions so far this year. A good travel agent will sort this for you; if they don't then keep pushing or speak to the line directly.
My overwhelming advice is just to do as much research in advance as you can and be prepared. Cuising is a super, super holiday for someone in a chair and I suspect you will take to it as well.