You've pretty much got it!
Some years ago, Monarch (I think it was...) did precisely this at LGW in respect of a programme of charter ops. As I recall BAA/GAL indicated that the airline would need to either "use 'em or lose 'em". I'm not sure what Monarch (if it was them) finally did.
I suspect that nowadays, the slots provisionally booked have to be confirmed by a specific cut-off date. As regards the BMI situation at LTN, apparently the slots during the 0700 to 0800 hour have all been confirmed and ACL on behalf of ACDL/LLAO offered/was going to offer a different time. However, the time differential between UK and Continental Europe meant that the arrival in Brussels of the first rotation would not have been as attractive because the service was aimed primarily at business users and so was designed to offer a full day's working in Brussels.
The main point that this illustrates is that arguably, for the first time ever, LTN has a period each weekday that is actually slot constrained (based on the existing hourly declaration of 32 and that as has been said previously, ACL for ACDL/LLAO still doesn't coordinate LTN corporate movements).
But one wonders, for how much longer...
