This will vary greatly from cruise to cruise, but usually there is a surcharge of between 30 to 70% on the normal fare. If you want to go on a specific cruise then you will probably end up paying a lot extra, but if you are open minded and scan the websites for last minute deals, then you may pick up a single fare for under £100 per day on a specific cruise. There are a few of these around at the moment. Happy cruising.
You don't have a friend who would like to join you. There are cruises where you can link up with someone you choose from a list provided. You phone/write to get to know who your famale companion is. It is a few years ago I heard about this and I think then it might have been Cunard or Norwegian Cruise lines. If they still do this it might be an option but you could also pick someone of course you don't get on with. Maybe someone else will pick up on this and advise you further.
Cruising as a single can be expensive, but there are two ways around having to pay massive single supplements. One is quite simply to register with a cruise club and wait until the late offers come through on the email, please note though that any late offers will invariably incur a 100% supplement for single occupancy, and nearly always be an inside cabin. The other choice is to book as soon as the cruises for the following year go on sale, this way you get the lowest supplements, and if you want to have an outside or balcony cabin this is the best time to be able to get one.
If you need to get away this year, try getting in touch with a Cruise Agent and I am sure they will be able to advise as to what is available now.
Best of luck and a happy return to cruising for you.