Jock Trapped. The reality is as a junior pilot on any fleet you will work most weekends. In the region of 90% and you will get 2 days off mid week. Probably Tuesday, Wednesday. I would expect you would see something like 5 on 2 off as a general pattern. You will get some 3 day blocks off but less so in the summer. You can expect to finish late and start early, at least it seems that way. JSS is a unknown but looking at my dry run results my rosters are far far harder under JSS. Try 16 days work with a weeks leave for example or a summer example had me working 20 days. 6 on 1 off, 5 on 2 off and repeat.
If you have a home base with a loco and command think very very carefully. I can not recommend it but if your drive is to one day fly long haul and you are willing to take a significant pay cut and reduction in lifestyle then go for it. BA has put huge strain on my family and if it were not for my wife being so understanding I would probably be another failed family story.
Just think very carefully at the pros and cons. Some love it, some hate it.