I'd be worried, if I was paying you £550 for the job.
If there was two of you, it would be quicker. So why bump the price up?
I'd have to say I'm with you on this one. I look at it like this
A job is worth £x, one man gets £x , two men get £x/2
I think in this case , snasty, if you are not confident skimming the ceiling on your own, then don't because you will not only make a mess of it but you will have a horrible time doing it, under pressure all the time
You may have made a mistake in your pricing, by not assessing the ceiling properly first, but you are in the situation now so lets look at what we can do.
Presuming you can cove it yourself in a day(one man-day), why not tack and skim it using a mate in one day (two man-days)
This way it is still three man days and you will have a much easier time boarding it, you can relax skimming it and have a doddle of a day coving it. And you have gained a bit of experience in pricing work.
You will make the same money per day, but have a day's less work. To me that it much better than going back on your price.
Now this may sound harsh, and I know it is unfair to lads who are starting out but when you are a plasterer, a 32 m ceiling is just an average hit. It is much easier than doing 30 m split up into three or four rooms.