Is It Worth Finishing Bronze and Silver DofE After Leaving School?

Sort of depends, really. From your post history it sounds like you're looking to apply to a lot of entry-level roles and apprenticeships that don't require specific qualifications. I'd say with that context it probably is worthwhile (especially if you currently have a small amount of previous work experience). If you reach a point where you have several years of work experience it starts being less worthwhile including it on your CV.
I think DofE can also sometimes be useful if applying to public sector roles i.e. teaching, working in a library (because volunteering is a mandatory element), as it shows willingness to be engaged with your community - but that's mostly speculation.
Your old school will have general contact details. You may just have to call the front desk reception and explain that you left the school recently - they may be able to give you an email for the assessors you need to contact (as long as you know their names!) - then hopefully they'll get back to you. Your sports section of Bronze could probably be signed off again by someone else as long as you had proper documentation for it, but the expedition would be harder.

Thanks for the reply. I think I’ll give it a shot since it makes sense — I’ve got some spare time, and I’m still in good contact with the school because I resat my Maths GCSE there this June.

For the Silver award, it makes sense to finish it off, and for Bronze, it’s just a matter of getting it signed off since I’ve already completed everything.

I might struggle a bit with the eDofE system, though, because my previous teachers recorded everything on paper rather than online. Hopefully, I can ask someone how to transfer that over, even though the staff who helped me before have now left the school.