Wings It's not voluntary in my school, we all do at least one residential per year (for 5 or 6 nights). The only person I've known not go in 5 years is someone whose mother was dying, and she had to take unpaid leave. Her mother died while we were on the trip. It's not written in our contracts but if we didn't go we'd have to take unpaid leave as all of the students go on a trip so there is no one left in school to teach, iyswim.
Most if not all TLRs also seem to include going on other residentials or weekend long activities with the students as and when needed (at least once a year), e.g. all HODs have to offer at least one residential enrichment trip, usually over a bank holiday weekend so as not to miss school time.
It was also not voluntary in my previous school - but everyone had a choice between ski trip or summer activities week.