What shape are you? First off, avoid anything with gathers or an elasticated waist (a small bit of elastic is fine, to ensure it fits snugly - what you don't need is masses of fabric to make you look even bigger). Go for something tailored and which skims you rather than engulfs you. Structure is also your friend if you have a large and wobbly tummy.
What are your lower legs like? (Be objective - are they like milk bottles or do they have some shape?) If they are not bad, go for one that hits you on the knee (or just below) and wear with heels.
Have you got a flattish bottom - are you reasonably narrow? If so, I would go for a straightish skirt, or one that fits and then flares out a bit - this can be quite flattering if you are tall (like the Evans one). If you are short, you might look a bit like a ship in full sail.
Or are you a pear shape, with a big bum? If so, again I think I might go for something fitted and straight, or maybe something bias cut to balance you out.
If you do go for floaty, again go for something that flares out and floats at the bottom, rather than something that floats from the waist.
I would have a look in M & S, Debenhams and Evans if you must (I am not a fan of Evans, although it does have its uses). Ann Harvey always makes me feel like I am going to start sparking with the amount of polyester, although I have had a couple of OK things from there. Do Dorothy Perkins go up to a size 22? You might have a look in there too. And have a look in John Lewis - Windsmoor stuff is good - although it is outside your budget, they often have stuff reduced, and I've bought skirts in there for 20-40 quid in the sale. Chesca (also in JL) can be a bit hit and miss - I got a great bandage skirt there in black. But a lot of the stuff is a bit "Mother of the Bride" (but more expensive polyester than Ann Harvey). H & M, if they still do their BiB range might have something good too.
Either go on your own or with an objective friend!