You are stressed by your current situation. When I am stressed, I try and remove the sources of stress, or make changes to my life, to improve personal wellbeing, to become happier. You may be suffering a little from burnout after doing a Bachelors and Masters degrees, and feel a more relaxing change is required, though normal jobs can be very stressful themselves. As others have said, you have the option of returning to a PhD at a later date. Yet, unhappiness with your the day-to-day work situation means that something has to change.

You cannot continue living with stress, therefore, what are your options? Leave the PhD or get changes to the current working situation. There are consequences to both. If you leave the PhD, depending upon your situation, you may need to leave the UK. Alternatively, changes to the current arrangements will require discussions with supervisors and University support staff, you are not the first person in this situation. They will help you.

If you decide to leave it may take a few weeks to go through due processes. A compromise, if your work is suitable, is to plan to leave after one year, enough time to sort out affairs, and not make your time in the UK a waste. I.e., see if you can convert to a Masters by Research. Having a closer endpoint on the horizon would give you something to focus on every day.

To answer your last question. No, you do not need to give back any salary paid.

Although you may not always be able to avoid difficult situations, you can modify the extent to which you can suffer by how you choose to respond to the situation. - Dalai Lama XIV

You are stressed by your current situation. When I am stressed, I try and remove the sources of stress, or make changes to my life, to improve personal wellbeing, to become happier. You may be suffering a little from burnout after doing a Bachelors and Masters degrees, and feel a more relaxing change is required, though normal jobs can be very stressful themselves. As others have said, you have the option of returning to a PhD at a later date. Yet, unhappiness with your day-to-day work situation means that something has to change.

You cannot continue living with stress, therefore, what are your options? Leave the PhD or get changes to the current working situation. There are consequences to both. If you leave the PhD, depending upon your situation, you may need to leave the UK. Alternatively, changes to the current arrangements will require discussions with supervisors and University support staff, you are not the first person in this situation. They will help you.

If you decide to leave it may take a few weeks to go through due processes. A compromise, if your work is suitable, is to plan to leave after one year, enough time to sort out affairs, and not make your time in the UK a waste. I.e., see if you can convert to a Masters by Research. Having a closer endpoint on the horizon would give you something to focus on every day.

To answer your last question. No, you do not need to give back any salary paid.

Although you may not always be able to avoid difficult situations, you can modify the extent to which you can suffer by how you choose to respond to the situation. - Dalai Lama XIV