I've used cash for clothes a couple of times, mainly for things like old bedding, curtains and towels. Like others have said it's only 50p per kg but when it's stuff you can't sell on EBay and not good enough for charity shops it seems like a good solution.
I've used music magpie a few times for clothes - stuff I know wouldn't sell on ebay, but still very good condition. They give about £1 per top/jumper, sometimes £1.80-£2 pair of jeans. You just enter the details on the website, they give you a price then tell you how many boxes to pack stuff into and labels to print. You pick a day for a pick-up (also get option of leaving boxes outside house if you're not around in daytime) then get a cheque a week of so later.
I got £8 for 16kg (50p/kg) which was great because it was stuff too tatty to sell and I'd have to walk to the charity shop which wasn't manageable with baby and toddler (and bags!) so we popped to the cash for clothes thing near us then went for lunch at the sainsburys cafe when we'd finished shopping. Nice unexpected treat.
You don't get masses of money, but there is only so much you can dump on charity shops before it gets embarrassing. I got about £8 last summer for about 4 bags. All you do is go in and dump the bags on massive scales and they hand over a the cash.
You can always give it to charity. Or buy the DC ice creams like I did