It's such a depressing situation, isn't it! I'm 
for you showmewine and 
at the greed and callousness of your employers.
I work as a private carer for an elderly lady, employed by her son to spend a couple of hours a day with her in her home. When I first started working with her, she was very confused, and shortly afterwards had to go into respite care in a local care home. I continued visiting her there every day, and from what I saw, that home was a very good one. There was a beautiful garden, and lots of communal areas, and the staff seemed friendly and to have time for the patients. That home was a private sector home, but with an allocated number of NHS beds. I don't know how the conditions were for the staff, but they all seemed cheerful and to have time to chat a bit.
Now that my lady is home again, she has carers(funded by social services, but provided by an agency)come in thrice daily to heat up the meals I've made for her, and to wash and dress her in the morning. I get to hear a lot about the care agency they work for, and there are good and bad aspects to their conditions. The money is about £11ph, but the hours are really long - they work 14 hour days, but with 3 hours off (1hr at lunch, 2hrs in the afternoon), six days a week. The carer I know most works 12 days on, 2 days off; it's really punishing. The agency is constantly short staffed, so they put massive pressure on them to work their days off or longer hours. I don't know why they don't employ part-time workers (obviously you can't work full time, from what you've said, showmethewine ), but probably other agencies do. The advantages of home care seem to be that they aren't being supervised all the time - obviously they are answerable to their manager, but they are fairly autonomous and seem well supported. This would probably be a much nicer environment to work in than what you're describing.
Also, have you considered doing bank HCA work? Healthcare Assistants work in hospitals and other healthcare settings, and the NHS has a 'bank' of staff (not just HCAs; nurses, all sorts of staff) on whom they call when they need to staff a shift. The advantage to that is that you usually get a higher hourly rate, and that you can tell them when you're available. You never know where you're going to be sent, but from what you've said about your working conditions, they wouldn't be hard to improve on! At least if you're working in the NHS, there are grievance procedures if you get treated like shit. Have you looked on the NHS careers website? They have a big jobs section.
Would a job like mine suit you? A couple of hours a day, flexible and fitted around the school run? I know I've been really lucky with mine (I was asked to do it, rather than looking for the job), but would it be worth advertising yourself as a private carer? I think a lot of elderly people could do with just a bit of extra help every day/few times a week. I certainly get enormous job satisfaction out of helping someone who needs it, but without taking all the crap you've been putting up with!
Good luck - I really mean that; the care industry needs a lot more people like you in it. 
Don't give up if you still have that urge to make a difference - change the home you work at, look along parallel lines at similar jobs etc, but find something that suits you and doesn't leave you so burned out that you lose the ability to care any more!