I (also now year 12) did work experience with my MP over summer year 11, so you could see if yours might be more willing to do it over the summer as well?
If not, you could always give a local combined authority a go, otherwise a local party association could be willing to give you some either on the ground campaigning or operations for a campaign - especially if you're a member, of course!
Hope that's helpful - I'm in the same boat as you in finding work experience in politics, so good luck!