I understand your predicament but cruise lines can charge extra because cabins for singles are normally three-quarter the size of doubles so it's not financially viable for them to charge a lot less. Many single women travelling as per your case would double up and although they are not "close" as you put it have you asked them their opinion or talked at length about it? Twin beds are in virtually every cabin. Failing that ...you sleep with your Mum and your husband with his.... just a thought.. Nei.
Hi, I have just had a look at the two cruises you have mentioned, both on the P&O website, and the Liverpool Cruise Club website (the people I usually book with). The single supplement on both is now 35%, which in my experience is not too bad. You will be hard pressed to beat the price of £1500 (OE Grade Inside Cabin) for a 14 night Mediterranean cruise. I know the single supplement seems unfair, but it is just a fact of life when it comes to cruising.
As for the weather in October, nobody can really say what it will be like. I have been over the Bay of Biscay many times, and experienced virtual flat calms to gales; however when you reach the Med usually the weather improves.
If you are going to book do it soon, in my experience as the cruise fills up with passengers, the single supplement gets higher and higher.
Please don't be put off by the time of year, or the perceived high cost, and treat both your mum's to a fine time.
A final tip, if you do go, and want then go on even more, book as early as possible, the single supplements are at their lowest as soon as the first edition brochure comes out, I know this can mean booking 12 to 20 months in advance, but the savings really are worthwhile.
Try Fred. Olsen. They have a reasonable number of single cabins. It's a bit difficult to get cheap offers on these but not impossibl.