Sounds like you've been unlucky, your Watch is running out of spec.
First make sure it is fully wound, hand wind if you need to by unscrewing the crown you will feel it pop slightly this is the winding position.
You cannot overwind an automatic.
Then set the time: https://www.timeanddate.com/worldclock/uk/london
Then time it over a 24 hour period.
I would estimate your particular Watch should be in the region of -2 seconds to +2 seconds a day, my twenty-year-old Seamaster SMP serviced 2 years ago can easily achieve +2 seconds a day, so no reason yours cannot.
The CoAxial movements run at 25,200 BPH or at least the Cal 2500C variants if I remember correctly, but this should have no real bearing on accuracy it is after all COSC certified.
Please keep us informed.
Rest assured you have a fine Watch when everything is sorted out.