An "unavoidable" is when the aircraft goes tech or u/s in some way or is delayed so that the links are missed, then you are likely to be accomodated. You appear to be on the scheduled timings, and are the ones you are booking. What you book and how you spend the transit time is pretty much personal choice. Usually the ticket cost reflects these planning considerations.

I regularly transit LHR overnight like this (twice this week!) because the last flight in/ first flight out does not allow a connection. This is just normal schedule issues.