This isn't a slots issue - it is a staffing issue. In order to obtain grandfather rights to a particular slot, you need to operate at least 80% of the slots held on a given day. For example, if you have a 15:00 slot on a Monday for a 20 week operating season, you need to operate 80% of the 20 weeks to get the grandfather rights. You can therefore cancel up to four flights over the course of that season before you lose your slot for the next year.
My understanding is that manpower levels have been cocked up and so they cannot crew certain long-haul trips. What has been happening is, for example, that today's BA093 LHR-YYZ has been cancelled (as far as passengers are concerned) but actually flew empty with flight crew and cargo aboard. Today's BA092 YYZ-LHR return flight operates just fine; and tomorrow's BA092 YYZ-LHR (which the same crew was due to operate back) is cancelled. And so on. Not great, but you've only buggered up two lots of passengers instead of four. Fundamentally bad news for the environment though, and this type of thing really doesn't help the entire aviation industry in general when plastered all over the evening news.