Hi there,
I am also based in the UK and was admitted to a rehab centre back in January.
The first couple of days were focused primarily around physical recovery - I was breathalysed and checked over by a doctor on arrival and my blood alcohol content was monitored until it dropped below a certain level, at which point I was given my first dose of Librium to detox. If you need a detox, they will do everything they can to make things as comfortable as possible for you. I stayed in my room for the entirety of the first full day, save for meeting my 'detox buddy' (everyone at my centre was assigned one during my stay) - someone who has been staying a little longer and is further down the recovery process than yourself, who is able to offer any words of encouragement and give you an idea of what to expect. There was no huge pressure for the first couple of days - I was encouraged to eat small amounts of food and gradually build this up, and go to the communal area to mingle if I felt well enough, but I wasn't expected to participate in the full programme.
My stay was for three weeks in total - as LBrain has said, we had a very strict timetable which did include early starts. A typical day may consist of some group therapy, CBT, holistic treatment or time for personal assignments/listening to the completed assignments of others (I completed my Step 1 whilst I was there, and a lot of people had to write a 'Life Story'). In the evenings, we might go to a gym/spa, have a quiz/games night, or attend an AA/NA meeting. Once a week, we had a guest speaker - usually another recovering alcoholic. Weekends were a bit more open-ended, but we might go out on an excursion, do some art therapy or go for a walk. We also received visitors on a Sunday afternoon.
You will learn a lot and it should be one of the most valuable experiences in your recovery. Everyone at my centre was really friendly, and we would all support/cheer each other on when times were hard. I am still in touch with a lady who went in a day or so after I did and we occasionally catch up at aftercare or NA meetings.