I supervise PhDs and I'm sorry you're so unhappy doing yours.
I would urge you not give up your place without a period of reflection and consultation. There are (and certainly used to be) some shockers of supervisors and I had one back in the day but most UK HEIs now have far more monitoring of the process. In my institution candidates only have supervision teams of two (occasionally three) supervisors and we are required to report to our graduate school at all points.
If you've passed your progression points and these reports or presentations were overseen by people who aren't your supervisors (they should have been) then you can have confidence - it shouldn't be possible to 'scrape' through, you're either up to standard or not - might your under-confidence be colouring this?
Thinking you don't have either the considered attention of your supervisors or good direction is obviously a problem and I would address that. You have put time and work into your PhD, you have collected your data, you will normally have up to four years to complete on a three-year funded project so you lose nothing by going to the body which supervises your supervisors (research school or similar), via your supervisors if you have a decent relationship, and ask to work in a different way.
You are not letting anyone down, they are not doing you a favour, it is their job to take you as far as they are able and if you don't complete, your work doesn't count for that institution, which is a black mark for them, so it is in their interests to help you as far as possible. I am very surprised you were unable to defer your studentship and take time out when you were ill, this in my experience has always been possible and I'd be bewildered by any rules which say you can't.
If you want permission to walk away then have no fears of doing that. It's a study project, nothing more or less, and unless you have your heart set on being an academic, it's an accessory to a career not an essential (and I work with plenty of academics who don't have 'study' PhDs. There are other routes to take such as PhD by publication of original work, Professional Doctorates and also you may be able to convert the work you have already done into an MPhil.
Supervisors should support you, they should have the relevant professional experience to do that, they should have PhDs themselves, but you will become more expert than them in your topic and there are normally a range of people in a given subject-area who could supervise you - in my experience it's very important that they are the right supervisors for you, there has to be a fit between you. Arts and Humanities PhDs are usually the worst for isolating students who think they must carry a huge 'life's work' burden themselves. It's just a research project, and your supervisors should be able to help you find the most pragmatic and time-efficient way of getting it done and off.
I think you should make your mind up to act, one way or another, because stewing away like this can only be harmful. It's an institution, it's impersonal, if you assert yourself to e.g. change your way of working (including supervision) or if you quit, the institution will go on the same tomorrow as today. Good luck!