Congrats on the place @ayahcip , William is a great person and educator, and he projects his energy onto the whole department.
I'm not sure JDs are only for those who aspire to become professional musicians - the violins often leave JDs for Menuhin if they aspire for a career. I'd say Guildhall (and JDs in general) is a good balance between music and other activities. Lots of violins go to selective academic schools - City, St Pauls etc, and they manage the academic and music workload so it is definitely doable.
Pressure will depend on the teacher, and we had all sorts of teachers at JD, from very nurturing to very pushy. Interestingly, both approaches worked for DD, just in different ways.
IMO the main advantage of JDs is high standard ensembles with children that are able to play challenging music and make really good sound. It's very rewarding for a musical person to be in a great ensemble. For violins, it's probably easier to find high level local groups but mostly chamber. Symphony orchestras for this age/level are few and far between. NCO is not a substitute for weekly ensembles - they are on holidays only, and for the younger ones they don't program challenging music as it can be tricky to rehearse in two days. Main orchestras are better but again, you need orchestral practice every week.
If you have access to a local string ensembles/symphony orchestras of high standard, then it's less of an advantage of course.
Other things to consider are music awareness classes, composition, Alexander technique, access to specialist physio etc - all in one place and focused on Guildhall students specifically.