I'm not British, but I'll try my best to answer the rest of your questions.
2. What happens in a psych ward is super variable and up to the people running it. In my area, the psych wards are actually hailed as respecting the patients at least in my city, but the horror stories about ones in other areas show that it's up to the people who actually run the place. Search up your nearest psych ward to see whether abuse was discussed, whether the food is cooked or will give you diarrhoea, etc.
3. Depends on your violence level and the people running it. Violent patients are isolated and stripped like the stereotypes of the psych ward you usually see. Non-violent patients are put into a cold/lukewarm room with the essentials to live as determined by law. Most countries don't take the time to make other comforts mandatory in their respective mental health acts, so after this point, you can only guarantee your basic human rights like food and water. The rest is up to the person running it (what kind of food is served, the temp of ac, how nice/rude the nurses are, etc.)
4. Depends if you are a voluntary patient or involuntary. I can't speak for British laws, but in places I do know, voluntary patients have way more leeway with what they can do. If you are involuntary (minor or deemed mentally challenged or a danger to themselves by a court) then you are only guaranteed to leave once you are cleared of suicidal ideation. Voluntary patients may leave against advice if they aren't actively in danger. Some areas of the world inflict a time period of 48-72 hours until a patient may decide to exit.
