Rights Under Mental Health Act

I started (through my own choice) going to an eating disorder support clinic a couple of weeks back, planning on a visit every two weeks. I needed a blood test, but she said it should be a one off hopefully.

For some background, I've wrestled with Anorexia for the past 5 years or so, gone down to 40kg, ended up in hospital, gained to 7oKg then lost it and am now at 39kg. I go to the gym every day for 1.5 hours and usually walk around an hour in addition.

Yesterday the GP went through the blood tests, which they need to increase to two/week and I need an ECG apparently. I'm 1.7m and drink around 10l water per day, so clearly my sodium was "unusual" but this is (in my opinion) explained. He mentioned that unless this improves, I should really attend hospital and he may need to section me under the mental health act.

Not going to the gym I'm finding to be the hardest part. I didn't go today and haven't been walking, I feel lethargic and lay having eaten more than normal. I desperately want this to work but would find it so much easier if I could just go, do my 40 minutes of weights to build, then eat like a horse to put muscle on - right now I feel like I'm gaining fat.

Is it really such a bad thing to go to the gym - even just to do weights - given I have a car and a desk job and can he legally section me under the mental health act like this?