Aleis, I wanted to respond to you, although I doubt I have anything helpful to suggest.
I am sorry you feel trapped, isolated, and that you have no time for your own life. I know those feelings all too well and many others here do also. (I'm an only child, my mother is 73 and has Alzheimer's-type dementia, probably for the past 3-5 years or more but only diagnosed in February. She was previously living alone with no help 100 miles from me and I was killing myself trying to see that she was all right. 11 months ago she was sectioned-or the US version of it-after being found in the cold, no coat, wandering, hallucinating and injured and then moved to a care home after it was determined she needed 24/7 supervision. She is now 10 miles from me and thank God is safe and fed and properly looked after.)
I'm also sorry to hear about all the practical difficulties you are having in trying to care for your mum and keep her independent. Dementia is just awful; I hate how it not only destroys the person with the disease, but also the rest of us as well.
I think that at a very minimum, your mum probably needs more visits from carers, and I am thinking daily. As I'm in the States and have a hazy understanding of how your system works, I don't know if you need an assessment, a call to Social Services, to involve the GP, or what, but someone else here will know and I hope will post soon to help you get that sorted.
There is also the idea of day care or a day centre, to get your mum out, fed, and socializing, and although I know it's difficult to access depending on where you are in the UK, possibly respite care?
I hope you will not discount the idea of a care home. I am not the only one here who was skeptical about how this would work (for many reasons, all of which I'm sure you can relate to!), especially since my mother, had she had capacity, would never have agreed.
But, I'm also not the only one here who has had an unexpectedly positive experience with having the PWD (person with dementia) move into a care home.
If anyone had told me that it could work out, and be the right thing to do for my mother, I may or may not have believed them, but it would have good for me if someone had said that to me. So, I am saying it to you.
I am not saying you must do this now, or next week, or next year, or ever, just that if and when it becomes a possibility, please do not dismiss it out of hand.
I am not sure I have practical suggestions about the situation with the phone, other than to ask if there is some way to install a handset that she can't unplug from the wall/otherwise disable. Sadly, I'm sure you understand that these sorts of problems are only going to get worse overall, although there will probably be times when she can use the phone and times when she cannot.
I am sure you have considered this, but how vulnerable do you feel your mum is from a safety standpoint? I'm thinking of whether it's time to disable the cooker, safeguard her finances, et cetera (apologies if you've mentioned this and I missed it).
I think Age UK and the Samaritans have people to listen and offer advice, and I'm rubbish at UK phone numbers but will go and look for those for you. If you get overwhelmed, don't hesitate to reach out. I do know that isolated, despondent feeling and am sorry you are experiencing this.
Age UK: 0800 169 2081
Samaritans: 116 123 (free from landline or mobile)
If I've fouled those numbers up, please will someone correct me? (And then you can taunt me for being an ignorant Yank.)
I wonder if you might find some relief or support from someone to talk to, or a carer's group, or something similar in your area.
Also please let me gently say that your kids are entitled to your time and attention, and that YOU are entitled to your own life. Please don't sacrifice yourself on the altar of dementia. As I'm sure you know, managing this disease will eat up all the energy and time that you give it, and never stop demanding more.
You're clearly a good person, who cares about her mother, and is trying to make sure her mother is safe and cared for. I agree that while you have the benefit of professional experience, it can be difficult, if not impossible, to be detached and apply that to your own situation. Please don't berate yourself or be too hard on yourself, if you can help it. Also please remember that none of this is your fault personally, nor is it a reflection of any failing on your part, personally or professionally. This is a fault of the disease. The enemy is dementia.
but wanted to let you know your post was being read. I think your idea of getting your mum's carers to involve her in what she feels she can do is a really good one. If it doesn't work out, it doesn't work out. [As long as no-one's killed of course 
