I have recently finished the last module with BGS and got an average of 98% over the 14 exams with distance learning so it can definitely be done. I spent 17 months on the two modules whilst having a full time job and trying to fly twice a week when possible.
Study is quite hard and I worked almost every night and at weekends. I had to split the modules over two years due to the two week brush up and one week exam per module using almost all my holiday entitlement each year.
Looking back I probably put too much work in but I wanted to do well and learn the subjects; however, I did do better than I expected.
The BGS course is very good and if you put the work in you will pass.