Inspections are literally just looking at things to make sure they're not broken. Pretty much another mini MOT, in between normal oil services.
I only pay for the inspections because I like the service book reading correctly. Don't think I'll bother next time though, it's simply not worth it on an older vehicle.
Regarding spark plugs on inspection 2. My indy says the spark plugs are changed, but on my invoice it has no mention of them being paid for, yet the type of oil used it on there, so either it's included in the price of the inspection, or they didn't change them last time.