You can chek if your vaccum advance is leaking simply by sucking on
the the little pipe that goes to the dizzy. Do this with the dizzy cap
off and you should see the base plate move. Don't expect it to be easy
you need to pull a fair bit, so if it's like sucking through an open
straw then you have a leak.
Your dizzy is designed to work with the vacuum advance. You could
blank off the vacuum unit but you'd really deen to change the advance
curve. If you simply blank it off you'll notice a considerable drop in
responsiveness  and low end torque. Cooper s's did not have a vacuum
unit but unless you plan to drive your engine continuously above
3500-4000 rpm i'd recommend keeping it or replacing it if faulty.

Hope this help