Having assisted a mildly hypothermic caver out from Swildons on a Sump 1 trip, telling people they only need a furry is dangerous. Some cavers will be fine in a T shirt. Some cavers will need a wetsuit or be at significant risk of hypothermia. A lot of cavers I know are not a Mendippian build and often get very very inappropriate advice about how much they need to wear on trips like Sump 1, Short Round, Upper Flood, Dan-yr-Ogof etc...